extensions/markdown-language-features/src/preview/preview.ts/MarkdownPreview/_updateImageWatchers
class MarkdownPreview:	private _updateImageWatchers(srcs: Set<string>) {
		// Delete stale file watchers.
		for (const [src, watcher] of this._fileWatchersBySrc) {
			if (!srcs.has(src)) {
				watcher.dispose();
				this._fileWatchersBySrc.delete(src);
			}
		}

		// Create new file watchers.
		const root = vscode.Uri.joinPath(this._resource, '../');
		for (const src of srcs) {
			const uri = urlToUri(src, root);
			if (uri && !MarkdownPreview._unwatchedImageSchemes.has(uri.scheme) && !this._fileWatchersBySrc.has(src)) {
				const watcher = vscode.workspace.createFileSystemWatcher(new vscode.RelativePattern(uri, '*'));
				watcher.onDidChange(() => {
					this.refresh(true);
				});
				this._fileWatchersBySrc.set(src, watcher);
			}
		}
	}