xinerama: Implement graphics exposures for window->pixmap copies (v4)

This code is using GetImage to accumulate a logical view of the window
image (since the windows will be clipped to their containing screen),
and then PutImage to load that back into the pixmap.  What it wasn't
doing was constructing a region for the obscured areas of the window and
emitting graphics exposures for same.

v2: Fix coordinate translation when the source is the root window
v3: Create sourceBox with the right coordinates initially instead of
translating (Keith Packard)
v4: Clamp the region to 15 bits to avoid overflow (Keith Packard)

Signed-off-by: Adam Jackson <ajax@redhat.com>