[Web] Small fixes and enhancements.

- "Definitive" fix for ENOENT randomly disappearing from emscripten.
- Proper shutdown when setup fails.
- Re-enable WebGL explicit buffer swap.
- Re-enable optional per-pixel transparency.
- Add type cast to make closure compiler happy.
- Remove emscripten Safari WebGL workaround.
- Improve AudioWorklet cleanup.

platform/web/js/engine/config.js

@@ -317,7 +317,8 @@ const InternalConfig = function (initConfig) { // eslint-disable-line no-unused-
 		if (!(this.canvas instanceof HTMLCanvasElement)) {
 			const nodes = document.getElementsByTagName('canvas');
 			if (nodes.length && nodes[0] instanceof HTMLCanvasElement) {
-				this.canvas = nodes[0];
+				const first = nodes[0];
+				this.canvas = /** @type {!HTMLCanvasElement} */ (first);
 			}
 			if (!this.canvas) {
 				throw new Error('No canvas found in page');