message: fix webgl build
  we need to select opengl on webgl.
diff: @@ -93,7 +93,9 @@ DefaultPlatform* DefaultPlatform::create(Backend* backend) noexcept {
  #endif
  if (*backend == Backend::DEFAULT) {
  -#if defined(ANDROID)
  +#if defined(__EMSCRIPTEN__)
  + *backend = Backend::OPENGL;
  +#elif defined(ANDROID)
  *backend = Backend::OPENGL;
  #elif defined(IOS) || defined(__APPLE__)
  *backend = Backend::METAL;
  