repeated parsing of file directories by every getImage call

[jcphill]

As shown below, every call to getImage() calls await this.parseFileDirectoriesPerFile() without using the result. parseFileDirectoriesPerFile() saves the result in this.fileDirectoriesPerFile but that variable is never referenced. The result is that every call to getImage() calls parseFileDirectoryAt() for every file for no reason.

geotiff.js/src/geotiff.js

Lines 613 to 634 in af0421b

	this.fileDirectoriesPerFile = null;
	this.fileDirectoriesPerFileParsing = null;
	this.imageCount = null;
	}
	async parseFileDirectoriesPerFile() {
	const requests = [this.mainFile.parseFileDirectoryAt(this.mainFile.firstIFDOffset)]
	.concat(this.overviewFiles.map((file) => file.parseFileDirectoryAt(file.firstIFDOffset)));
	this.fileDirectoriesPerFile = await Promise.all(requests);
	return this.fileDirectoriesPerFile;
	}
	/**
	* Get the n-th internal subfile of an image. By default, the first is returned.
	*
	* @param {number} [index=0] the index of the image to return.
	* @returns {Promise<GeoTIFFImage>} the image at the given index
	*/
	async getImage(index = 0) {
	await this.getImageCount();
	await this.parseFileDirectoriesPerFile();

[jcphill]

Just to note, this only affects the MultiGeoTIFF class (overviews in separate files). The single-file case looks OK.