[HTML5] JS callback functions now returns passed value.

JavaScript callbacks created via the `JavaScript.create_callback` method
used to always return void.

With this patch they return the value returned by the Godot function as
one would expect.

platform/javascript/javascript_singleton.cpp

@@ -54,6 +54,7 @@ extern int godot_js_wrapper_object_setvar(int p_id, int p_key_type, godot_js_wra
 extern void godot_js_wrapper_object_set(int p_id, const char *p_name, int p_type, godot_js_wrapper_ex *p_val);
 extern void godot_js_wrapper_object_unref(int p_id);
 extern int godot_js_wrapper_create_cb(void *p_ref, void (*p_callback)(void *p_ref, int p_arg_id, int p_argc));
+extern void godot_js_wrapper_object_set_cb_ret(int p_type, godot_js_wrapper_ex *p_val);
 extern int godot_js_wrapper_create_object(const char *p_method, void **p_args, int p_argc, GodotJSWrapperVariant2JSCallback p_variant2js_callback, godot_js_wrapper_ex *p_cb_rval, void **p_lock, GodotJSWrapperFreeLockCallback p_lock_callback);
 };
 
@@ -257,6 +258,16 @@ void JavaScriptObjectImpl::_callback(void *p_ref, int p_args_id, int p_argc) {
 	Callable::CallError err;
 	Variant ret;
 	obj->_callable.call(argv, 1, ret, err);
+
+	// Set return value
+	godot_js_wrapper_ex exchange;
+	void *lock = nullptr;
+	const Variant *v = &ret;
+	int type = _variant2js((const void **)&v, 0, &exchange, &lock);
+	godot_js_wrapper_object_set_cb_ret(type, &exchange);
+	if (lock) {
+		_free_lock(&lock, type);
+	}
 }
 
 Ref<JavaScriptObject> JavaScript::create_callback(const Callable &p_callable) {

platform/javascript/js/libs/library_godot_javascript_singleton.js

@@ -34,6 +34,7 @@ const GodotJSWrapper = {
 	$GodotJSWrapper__postset: 'GodotJSWrapper.proxies = new Map();',
 	$GodotJSWrapper: {
 		proxies: null,
+		cb_ret: null,
 
 		MyProxy: function (val) {
 			const id = IDHandler.add(this);
@@ -202,15 +203,27 @@ const GodotJSWrapper = {
 		let id = 0;
 		const cb = function () {
 			if (!GodotJSWrapper.get_proxied_value(id)) {
-				return;
+				return undefined;
 			}
+			// The callback will store the returned value in this variable via
+			// "godot_js_wrapper_object_set_cb_ret" upon calling the user function.
+			// This is safe! JavaScript is single threaded (and using it in threads is not a good idea anyway).
+			GodotJSWrapper.cb_ret = null;
 			const args = Array.from(arguments);
 			func(p_ref, GodotJSWrapper.get_proxied(args), args.length);
+			const ret = GodotJSWrapper.cb_ret;
+			GodotJSWrapper.cb_ret = null;
+			return ret;
 		};
 		id = GodotJSWrapper.get_proxied(cb);
 		return id;
 	},
 
+	godot_js_wrapper_object_set_cb_ret__sig: 'vii',
+	godot_js_wrapper_object_set_cb_ret: function (p_val_type, p_val_ex) {
+		GodotJSWrapper.cb_ret = GodotJSWrapper.variant2js(p_val_type, p_val_ex);
+	},
+
 	godot_js_wrapper_object_getvar__sig: 'iiii',
 	godot_js_wrapper_object_getvar: function (p_id, p_type, p_exchange) {
 		const obj = GodotJSWrapper.get_proxied_value(p_id);