Patches for bugs #7555 and #8485

[James Supancic]

Patches for bugs #7555 and #8485

I created a patch that fixes bug #7555 (Failure for Xdmx to properly
set clip rectangles) and bug #8485 (Lack of addremovescreen support in
Xdmx when the RENDER extension is enabled). I have tested the patch
and it works fine for me. Please review, and report problems or (in
the lack of problems) commit to head.

Thank you for your time,
James Steven Supancic III