Transparent window background (desktop-only)

[chicoxyzzy]

• macOS
• Linux
• Windows

[chicoxyzzy]

I'm trying to add this on Windows

    if (opts.transparent) {
      CORE2WEBVIEW_COLOR color = { 0 };
      webview->put_DefaultBackgroundColor(color);
    }

it compiles fine, without errors in the app, but it doesn't work for me

I took example code from here
https://github.com/MicrosoftEdge/WebView2Samples/blob/85e53c9dc4245ed74fc737554640644d4b0f3bb5/SampleApps/WebView2APISample/ViewComponent.cpp#L84
and it looks correct acccording to what I see in the webview2 win32 c++ api

[chicoxyzzy]

TODO: add tests for JS API

[chicoxyzzy]

Should I rebase the branch? cc @heapwolf @jwerle