VideoOutputOpenGL: Fix embedded playback when using OpenGL for main UI

Now that OpenGL video will share the OpenGL context with the main UI,
setting the mask (in guidgrid.cpp) effectively hides the embedded video
- so clear the mask.

mythtv/libs/libmythtv/videoout_opengl.cpp

@@ -506,8 +506,13 @@ void VideoOutputOpenGL::PrepareFrame(VideoFrame *buffer, FrameScanType t,
         gl_context->SetBackground(0, 0, 0, 255);
     gl_context->ClearFramebuffer();
 
-    if (gl_context->IsShared() && GetMythMainWindow() && window.IsEmbedding())
-        GetMythMainWindow()->draw();
+    MythMainWindow *mwnd = GetMythMainWindow();
+    if (gl_context->IsShared() && mwnd && window.IsEmbedding())
+    {
+        if (mwnd->GetPaintWindow())
+            mwnd->GetPaintWindow()->setMask(QRegion());
+        mwnd->draw();
+    }
 
     if (gl_videochain)
     {