gstreamer media backend: Disable OpenGL on Windows

...for at least WGL, since we are not allowed to use wglMakeCurrent(),
which is eventually called by gdk_gl_context_make_current() to share WGL
contexts across different threads, which will cause a crash.

This means, we cannot enable WGL in the gstreamer media backend on
Windows.

modules/media/gtkgstpaintable.c

@@ -118,6 +118,18 @@ gtk_gst_paintable_video_renderer_create_video_sink (GstPlayerVideoRenderer *rend
   GtkGstPaintable *self = GTK_GST_PAINTABLE (renderer);
   GstElement *sink, *glsinkbin;
 
+#if GST_GL_HAVE_WINDOW_WIN32 && GST_GL_HAVE_PLATFORM_WGL && defined (GDK_WINDOWING_WIN32)
+  /*
+   * Unfortunately, we can't connect the GstGLContext with our GDKGLContext,
+   * since gdk_gl_context_make_current(), which calls wglMakeCurrent(), does not
+   * allow us to share WGL contexts across threads, which will cause a crash.
+   * See MR !3034, so no WGL in the gstreamer media backend :(
+   */
+  sink = g_object_new (GTK_TYPE_GST_SINK,
+                       "paintable", self,
+                       NULL);
+  return sink;
+#else
   sink = g_object_new (GTK_TYPE_GST_SINK,
                        "paintable", self,
                        "gl-context", self->context,
@@ -131,6 +143,7 @@ gtk_gst_paintable_video_renderer_create_video_sink (GstPlayerVideoRenderer *rend
   g_object_set (glsinkbin, "sink", sink, NULL);
 
   return glsinkbin;
+#endif
 }
 
 static void