wayland: Prevent stale paints and weird artifacts when using Weston
Weston releases buffers almost immediately after they're done, which means that GTK+ doesn't use a temporary surface and instead paints directly onto the SHM backing store that Weston will use. Normally, after painting to the temporary surface, GTK+ *replaces* the existing backing surface with CAIRO_OPERATOR_SOURCE. However, if we immediately paint to the backing surface, it might have junk from the last paint in it. So clear out the backing surface whenever somebody calls begin_paint_region(). Maybe we should just always use the temporary surface like the X11 codepath, since that prevents us from having to do weird things like this, but oh well.
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