gltexture: Only wait if we need to

If the texture was created in the same context
we are downloading it from, then waiting on the
sync will be a no-op.

gdk/gdkgltexture.c

@@ -102,7 +102,7 @@ gdk_gl_texture_invoke_callback (gpointer data)
 
   gdk_gl_context_make_current (context);
 
-  if (invoke->self->sync)
+  if (invoke->self->sync && context != invoke->self->context)
     glWaitSync (invoke->self->sync, 0, GL_TIMEOUT_IGNORED);
 
   glBindTexture (GL_TEXTURE_2D, invoke->self->id);