xorg updates

I'm getting the x-updates branch closer to merging (to master). After some research I thought that we should probably choose the 1.13 branch of xorg-server. Here's the change. Ideas welcome. My machine now builds a superset of xorg modules than in master :-)

- update pixman to later, cairo-preferred version
- update xorgserver to 1.13.4
- update some modules to work with the newer server
- fix many other modules via overrides
- huge cleanup in overrides via better propagation
  and pixman include flattening
- URLs of XCB stuff have been moved