[HTML5] Add WebGL2 (GLES3) support using the OpenGL renderer.

Note, the editor build requires the mbedtls module to be manually
enabled, as it is currently needed as a ResourceUID dependency.

This will need to be addressed in a separate PR.

platform/javascript/os_javascript.h

@@ -90,6 +90,7 @@ public:
 	String get_user_data_dir() const override;
 
 	bool is_userfs_persistent() const override;
+	bool is_single_window() const override { return true; }
 
 	void alert(const String &p_alert, const String &p_title = "ALERT!") override;