Added cache for all resource requests

This eliminates the constant redownloading of referenced fonts images and such. The cache is cleared after the completion of  the toSvg  method and is based on the requested URL, not the cache-busted URL. Based on the work in  tsayen#336 and resolves #79

Author: IDisposable

src/dom-to-image-more.js

@@ -30,6 +30,7 @@
             images: images,
             util: util,
             inliner: inliner,
+            urlCache: [],
             options: {}
         }
     };
@@ -80,7 +81,13 @@
                     options.width || util.width(node),
                     options.height || util.height(node)
                 );
-            });
+            })
+            .then(clearCache);
+
+        function clearCache(result) {
+            domtoimage.impl.urlCache = [];
+            return result;
+        }
 
         function applyOptions(clone) {
             if (options.bgcolor) { clone.style.backgroundColor = options.bgcolor; }
@@ -568,68 +575,83 @@
         }
 
         function getAndEncode(url) {
-            if (domtoimage.impl.options.cacheBust) {
-                // Cache bypass so we dont have CORS issues with cached images
-                // Source: https://developer.mozilla.org/en/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest#Bypassing_the_cache
-                url += ((/\?/).test(url) ? "&" : "?") + (new Date()).getTime();
-            }
+            let cacheEntry = domtoimage.impl.urlCache.find(function (el) {
+                return el.url === url;
+            });
 
-            return new Promise(function (resolve) {
-                const httpTimeout = domtoimage.impl.options.httpTimeout;
-                const request = new XMLHttpRequest();
+            if (!cacheEntry) {
+                cacheEntry = {
+                    url: url,
+                    promise: null
+                };
+                domtoimage.impl.urlCache.push(cacheEntry);
+            }
 
-                request.onreadystatechange = done;
-                request.ontimeout = timeout;
-                request.responseType = 'blob';
-                request.timeout = httpTimeout;
-                if (domtoimage.impl.options.useCredentials) {
-                    request.withCredentials = true;
+            if (cacheEntry.promise === null) {
+                if (domtoimage.impl.options.cacheBust) {
+                    // Cache bypass so we dont have CORS issues with cached images
+                    // Source: https://developer.mozilla.org/en/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest#Bypassing_the_cache
+                    url += ((/\?/).test(url) ? "&" : "?") + (new Date()).getTime();
                 }
-                request.open('GET', url, true);
-                request.send();
-
-                let placeholder;
-                if (domtoimage.impl.options.imagePlaceholder) {
-                    const split = domtoimage.impl.options.imagePlaceholder.split(/,/);
-                    if (split && split[1]) {
-                        placeholder = split[1];
+
+                cacheEntry.promise = new Promise(function (resolve) {
+                    const httpTimeout = domtoimage.impl.options.httpTimeout;
+                    const request = new XMLHttpRequest();
+
+                    request.onreadystatechange = done;
+                    request.ontimeout = timeout;
+                    request.responseType = 'blob';
+                    request.timeout = httpTimeout;
+                    if (domtoimage.impl.options.useCredentials) {
+                        request.withCredentials = true;
+                    }
+                    request.open('GET', url, true);
+                    request.send();
+
+                    let placeholder;
+                    if (domtoimage.impl.options.imagePlaceholder) {
+                        const split = domtoimage.impl.options.imagePlaceholder.split(/,/);
+                        if (split && split[1]) {
+                            placeholder = split[1];
+                        }
                     }
-                }
 
-                function done() {
-                    if (request.readyState !== 4) { return; }
+                    function done() {
+                        if (request.readyState !== 4) { return; }
 
-                    if (request.status !== 200) {
+                        if (request.status !== 200) {
+                            if (placeholder) {
+                                resolve(placeholder);
+                            } else {
+                                fail(`cannot fetch resource: ${url}, status: ${request.status}`);
+                            }
+
+                            return;
+                        }
+
+                        const encoder = new FileReader();
+                        encoder.onloadend = function () {
+                            const content = encoder.result.split(/,/)[1];
+                            resolve(content);
+                        };
+                        encoder.readAsDataURL(request.response);
+                    }
+
+                    function timeout() {
                         if (placeholder) {
                             resolve(placeholder);
                         } else {
-                            fail(`cannot fetch resource: ${url}, status: ${request.status}`);
+                            fail(`timeout of ${httpTimeout}ms occured while fetching resource: ${url}`);
                         }
-
-                        return;
                     }
 
-                    const encoder = new FileReader();
-                    encoder.onloadend = function () {
-                        const content = encoder.result.split(/,/)[1];
-                        resolve(content);
-                    };
-                    encoder.readAsDataURL(request.response);
-                }
-
-                function timeout() {
-                    if (placeholder) {
-                        resolve(placeholder);
-                    } else {
-                        fail(`timeout of ${httpTimeout}ms occured while fetching resource: ${url}`);
+                    function fail(message) {
+                        console.error(message);
+                        resolve('');
                     }
-                }
-
-                function fail(message) {
-                    console.error(message);
-                    resolve('');
-                }
-            });
+                });
+            }
+            return cacheEntry.promise;
         }
 
         function dataAsUrl(content, type) {