Accept non-default main packs in engine.js startGame()

Allows using startGame() with main packs exported as .zip, but also any
other custom extension, for example if a web game host does not allow
the .pck filename extension.

misc/dist/html/default.html

@@ -229,7 +229,7 @@ $GODOT_HEAD_INCLUDE
 
 		(function() {
 
-			const BASENAME = '$GODOT_BASENAME';
+			const MAIN_PACK = '$GODOT_BASENAME.pck';
 			const DEBUG_ENABLED = $GODOT_DEBUG_ENABLED;
 			const INDETERMINATE_STATUS_STEP_MS = 100;
 
@@ -380,7 +380,7 @@ $GODOT_HEAD_INCLUDE
 			} else {
 				setStatusMode('indeterminate');
 				engine.setCanvas(canvas);
-				engine.startGame(BASENAME + '.pck').then(() => {
+				engine.startGame(MAIN_PACK).then(() => {
 					setStatusMode('hidden');
 					initializing = false;
 				}, displayFailureNotice);

platform/javascript/engine.js

@@ -130,13 +130,17 @@
 		this.startGame = function(mainPack) {
 
 			executableName = getBaseName(mainPack);
+			var mainArgs = [];
+			if (!getPathLeaf(mainPack).endsWith('.pck')) {
+				mainArgs = ['--main-pack', getPathLeaf(mainPack)];
+			}
 			return Promise.all([
 				// Load from directory,
 				this.init(getBasePath(mainPack)),
 				// ...but write to root where the engine expects it.
 				this.preloadFile(mainPack, getPathLeaf(mainPack))
 			]).then(
-				Function.prototype.apply.bind(synchronousStart, this, [])
+				Function.prototype.apply.bind(synchronousStart, this, mainArgs)
 			);
 		};