compositor: Preserve content from early-destroyed SHM buffers
If an SHM buffer is destroyed between being attached and repaint, we may never have its content available to the GL renderer, because we delay SHM uploads until repaint.
Try to ameliorate this by forcing damage to be flushed on destroy for SHM buffers.
Signed-off-by: Daniel Stone daniels@collabora.com Fixes: #604