The difference between the two is that XF86 has the clip helper that
lets you upload less data when rendering video that's clipped.  I
don't think that's really worth the trouble, especially in a world of
compositors, so I've dropped it to get to shared code.

It turns out the clipping code was broken on xf86-video-intel anyway.
To reproduce, run without a compositor, and use another window to clip
the top half of your XV output on the glamor XV adaptor: the rendering
got confused about which half of the window was being drawn to.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>