xcb: Be smarter about how we flush

For the remote X case the backing store previously always reuploaded
image data for every expose event. Instead of doing that create a remote
X pixmap and only flush repainted regions. For regular expose just copy
from the pixmap.

Additionally, atomically update the window by setting a clip mask and
flushing the entire region at once instead of doing it rect by rect.

Change-Id: I26bb1834b159e309c7ad93287dd297769f7e2633
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>