UWP: Fix build with Google ANGLE not supporting EGL_ANGLE_DISPLAY_ALL…

…OW_RENDER_TO_BACK_BUFFER

EGL_ANGLE_DISPLAY_ALLOW_RENDER_TO_BACK_BUFFER is not present in Google ANGLE,
and Microsoft ANGLE is outdated and not supported anymore.

platform/uwp/context_egl_uwp.cpp

@@ -118,10 +118,12 @@ Error ContextEGL_UWP::initialize() {
  EGL_PLATFORM_ANGLE_TYPE_ANGLE,
  EGL_PLATFORM_ANGLE_TYPE_D3D11_ANGLE,
 
+#ifdef EGL_ANGLE_DISPLAY_ALLOW_RENDER_TO_BACK_BUFFER
  // EGL_ANGLE_DISPLAY_ALLOW_RENDER_TO_BACK_BUFFER is an optimization that can have large performance benefits on mobile devices.
  // Its syntax is subject to change, though. Please update your Visual Studio templates if you experience compilation issues with it.
  EGL_ANGLE_DISPLAY_ALLOW_RENDER_TO_BACK_BUFFER,
  EGL_TRUE,
+#endif
 
  // EGL_PLATFORM_ANGLE_ENABLE_AUTOMATIC_TRIM_ANGLE is an option that enables ANGLE to automatically call
  // the IDXGIDevice3::Trim method on behalf of the application when it gets suspended.