[HTML5] Libraries refactor for linting.

Initial work to make liniting easier.

This includes:
- Rename http_request.js to library_godot_http_request.js.
- Rename externs.js to engine.externs.js.
- New library_godot_runtime.js (GodotRuntime) wraps around emscripten
  functions.
- Refactor of XMLHttpRequest handler in engine/preloader.js.
- Few fixes to bugs spotted by early stage linting.

platform/javascript/js/libs/library_godot_runtime.js

@@ -0,0 +1,120 @@
+/*************************************************************************/
+/*  library_godot_runtime.js                                             */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2020 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2020 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+const GodotRuntime = {
+	$GodotRuntime: {
+		/*
+		 * Functions
+		 */
+		get_func: function(ptr) {
+			return wasmTable.get(ptr); // eslint-disable-line no-undef
+		},
+
+		/*
+		 * Prints
+		 */
+		error: function() {
+			err.apply(null, Array.from(arguments)); // eslint-disable-line no-undef
+		},
+
+		print: function() {
+			out.apply(null, Array.from(arguments)); // eslint-disable-line no-undef
+		},
+
+		/*
+		 * Memory
+		 */
+		malloc: function(p_size) {
+			return _malloc(p_size); // eslint-disable-line no-undef
+		},
+
+		free: function(p_ptr) {
+			_free(p_ptr); // eslint-disable-line no-undef
+		},
+
+		getHeapValue: function (p_ptr, p_type) {
+			return getValue(p_ptr, p_type); // eslint-disable-line no-undef
+		},
+
+		setHeapValue: function(p_ptr, p_value, p_type) {
+			setValue(p_ptr, p_value, p_type); // eslint-disable-line no-undef
+		},
+
+		heapSub: function(p_heap, p_ptr, p_len) {
+			const bytes = p_heap.BYTES_PER_ELEMENT;
+			return p_heap.subarray(p_ptr / bytes, p_ptr / bytes + p_len);
+		},
+
+		heapCopy: function(p_heap, p_ptr, p_len) {
+			const bytes = p_heap.BYTES_PER_ELEMENT;
+			return p_heap.slice(p_ptr / bytes, p_ptr / bytes + p_len);
+		},
+
+		/*
+		 * Strings
+		 */
+		parseString: function(p_ptr) {
+			return UTF8ToString(p_ptr); // eslint-disable-line no-undef
+		},
+
+		strlen: function(p_str) {
+			return lengthBytesUTF8(p_str); // eslint-disable-line no-undef
+		},
+
+		allocString: function(p_str) {
+			const length = GodotRuntime.strlen(p_str)+1;
+			const c_str = GodotRuntime.malloc(length);
+			stringToUTF8(p_str, c_str, length); // eslint-disable-line no-undef
+			return c_str;
+		},
+
+		allocStringArray: function(p_strings) {
+			const size = p_strings.length;
+			const c_ptr = GodotRuntime.malloc(size * 4);
+			for (let i = 0; i < size; i++) {
+				HEAP32[(c_ptr >> 2) + i] = GodotRuntime.allocString(p_strings[i]);
+			}
+			return c_ptr;
+		},
+
+		freeStringArray: function(p_ptr, p_len) {
+			for (let i = 0; i < p_len; i++) {
+				GodotRuntime.free(HEAP32[(p_ptr >> 2) + i]);
+			}
+			GodotRuntime.free(p_ptr);
+		},
+
+		stringToHeap: function (p_str, p_ptr, p_len) {
+			return stringToUTF8Array(p_str, HEAP8, p_ptr, p_len); // eslint-disable-line no-undef
+		},
+	},
+};
+autoAddDeps(GodotRuntime, "$GodotRuntime");
+mergeInto(LibraryManager.library, GodotRuntime);