[HTML5] Refactor JS, threads support, closures.

- Refactored the Engine code, splitted across files.
- Use MODULARIZE option to build emscripten code into it's own closure.
- Enable lto support (saves ~2MiB in release).
- Enable optional closure compiler pass for JS and generated code.
- Enable optional pthreads support.
- Can now build with tools=yes (not much to see yet).
- Dropped some deprecated code for older toolchains.

platform/javascript/os_javascript.cpp

@@ -935,6 +935,7 @@ Error OS_JavaScript::initialize(const VideoMode &p_desired, int p_video_driver,
 	if (p_desired.fullscreen) {
 		/* clang-format off */
 		EM_ASM({
+			const canvas = Module.canvas;
 			(canvas.requestFullscreen || canvas.msRequestFullscreen ||
 				canvas.mozRequestFullScreen || canvas.mozRequestFullscreen ||
 				canvas.webkitRequestFullscreen