composite: Fix COW creation for Xinerama (v2)

composite: Fix COW creation for Xinerama (v2)

Say you have two 800x600 screens left/right of each other.  A window
that's 200x200 at +700+0 in protocol coordinate space will appear to be
at -100+0 in the coordinate space of the right hand screen.  Put another
way: windows are in the coordinate space of their root window pixmap.

We weren't doing this translation for the COW, so when rendering came in
to it you'd see the top-left chunk of the COW on all screens.  Cool
effect and all, but wrong.

v2: Only translate when Xinerama is active [keithp]

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>