The X11 queue_translation operation uses NextRequest to get the serial
of the XCopyArea operation where the translation should end. However,
if the gc passed to gdk_draw_drawable has a non-flushed clip region
(which it commonly has now for the window clipping) then the next
operation will be the GC flush, not the XCopyArea.

To handle this right we now pass in the GC to be used to
queue_translation and ensure that it is flushed before calling
NextRequest().