sna: Ensure we flush SHM pixmaps if rendering to a GPU shadow

sna: Ensure we flush SHM pixmaps if rendering to a GPU shadow

Normally, we try to render into the CPU bo of a SHM pixmap. However, under
the right circumstances we may try to do a series of rendering into the
GPU bo and then copy it back to the CPU bo. In that case, we need to be
sure to mark the pixmap as requiring the flush.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>