Small adjustments to run KWin on top of llvmpipe. Just dumped here to have the patch somewhere if someone else wants to try it and to be able to try again with newer mesa versions.

Tested with mesa 8.0 from debian experimental.

Compositing is worked, but just as a slide show (5 to 10 fps) and blocking one core completely. System completely unusable. Because of that: commit embargo.

Will try again once mesa 8 has migrated to debian testing.