Check only for WebGL 1.0, move test to HTML file

Whether to use WebGL 1.0 or 2.0 can only be determined at runtime after
reading project settings, so check for the lower version.

The test is now in the HTML file, so if desired WebGL 2.0 can be
checked early by changing the behaviour there.

platform/javascript/os_javascript.cpp

@@ -444,6 +444,7 @@ Error OS_JavaScript::initialize(const VideoMode &p_desired, int p_video_driver,
 			break;
 	}
 	EMSCRIPTEN_WEBGL_CONTEXT_HANDLE ctx = emscripten_webgl_create_context(NULL, &attributes);
+	ERR_EXPLAIN("WebGL " + itos(attributes.majorVersion) + ".0 not available");
 	ERR_FAIL_COND_V(emscripten_webgl_make_context_current(ctx) != EMSCRIPTEN_RESULT_SUCCESS, ERR_UNAVAILABLE);
 
 	video_mode = p_desired;