[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.
- Optional closure compiler run for JS and generated code.
- Enable lto support (saves ~2MiB in release).
- Can now build with tools=yes (not much to see yet).
- Dropped some deprecated code for older toolchains.
- Add onExit, and onExecute JS function.
- Add files drag and drop support.
- Add support for low precessor usage mode (via offscreen render, swap).

platform/javascript/engine/preloader.js

@@ -0,0 +1,139 @@
+var Preloader = /** @constructor */ function() {
+
+	var DOWNLOAD_ATTEMPTS_MAX = 4;
+	var progressFunc = null;
+	var lastProgress = { loaded: 0, total: 0 };
+
+	var loadingFiles = {};
+	this.preloadedFiles = [];
+
+	function loadXHR(resolve, reject, file, tracker) {
+		var xhr = new XMLHttpRequest;
+		xhr.open('GET', file);
+		if (!file.endsWith('.js')) {
+			xhr.responseType = 'arraybuffer';
+		}
+		['loadstart', 'progress', 'load', 'error', 'abort'].forEach(function(ev) {
+			xhr.addEventListener(ev, onXHREvent.bind(xhr, resolve, reject, file, tracker));
+		});
+		xhr.send();
+	}
+
+	function onXHREvent(resolve, reject, file, tracker, ev) {
+
+		if (this.status >= 400) {
+
+			if (this.status < 500 || ++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
+				reject(new Error("Failed loading file '" + file + "': " + this.statusText));
+				this.abort();
+				return;
+			} else {
+				setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
+			}
+		}
+
+		switch (ev.type) {
+			case 'loadstart':
+				if (tracker[file] === undefined) {
+					tracker[file] = {
+						total: ev.total,
+						loaded: ev.loaded,
+						attempts: 0,
+						final: false,
+					};
+				}
+				break;
+
+			case 'progress':
+				tracker[file].loaded = ev.loaded;
+				tracker[file].total = ev.total;
+				break;
+
+			case 'load':
+				tracker[file].final = true;
+				resolve(this);
+				break;
+
+			case 'error':
+				if (++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
+					tracker[file].final = true;
+					reject(new Error("Failed loading file '" + file + "'"));
+				} else {
+					setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
+				}
+				break;
+
+			case 'abort':
+				tracker[file].final = true;
+				reject(new Error("Loading file '" + file + "' was aborted."));
+				break;
+		}
+	}
+
+	this.loadPromise = function(file) {
+		return new Promise(function(resolve, reject) {
+			loadXHR(resolve, reject, file, loadingFiles);
+		});
+	}
+
+	this.preload = function(pathOrBuffer, destPath) {
+		if (pathOrBuffer instanceof ArrayBuffer) {
+			pathOrBuffer = new Uint8Array(pathOrBuffer);
+		} else if (ArrayBuffer.isView(pathOrBuffer)) {
+			pathOrBuffer = new Uint8Array(pathOrBuffer.buffer);
+		}
+		if (pathOrBuffer instanceof Uint8Array) {
+			this.preloadedFiles.push({
+				path: destPath,
+				buffer: pathOrBuffer
+			});
+			return Promise.resolve();
+		} else if (typeof pathOrBuffer === 'string') {
+			var me = this;
+			return this.loadPromise(pathOrBuffer).then(function(xhr) {
+				me.preloadedFiles.push({
+					path: destPath || pathOrBuffer,
+					buffer: xhr.response
+				});
+				return Promise.resolve();
+			});
+		} else {
+			throw Promise.reject("Invalid object for preloading");
+		}
+	};
+
+	var animateProgress = function() {
+
+		var loaded = 0;
+		var total = 0;
+		var totalIsValid = true;
+		var progressIsFinal = true;
+
+		Object.keys(loadingFiles).forEach(function(file) {
+			const stat = loadingFiles[file];
+			if (!stat.final) {
+				progressIsFinal = false;
+			}
+			if (!totalIsValid || stat.total === 0) {
+				totalIsValid = false;
+				total = 0;
+			} else {
+				total += stat.total;
+			}
+			loaded += stat.loaded;
+		});
+		if (loaded !== lastProgress.loaded || total !== lastProgress.total) {
+			lastProgress.loaded = loaded;
+			lastProgress.total = total;
+			if (typeof progressFunc === 'function')
+				progressFunc(loaded, total);
+		}
+		if (!progressIsFinal)
+			requestAnimationFrame(animateProgress);
+	}
+	this.animateProgress = animateProgress; // Also exposed to start it.
+
+	this.setProgressFunc = function(callback) {
+		progressFunc = callback;
+	}
+};