tgsi: add caps for fragment coord conventions (v3)

tgsi: add caps for fragment coord conventions (v3)

Changes in v3:
- Renumber caps to accomodate caps to add to master in the meantime
- Document caps
- Add unsupported caps to *_screen.c too

Changes in v2:
- Split for properties patch
- Use positive caps instead of negative caps

This adds 4 caps to indicate support of each of the fragment coord
conventions.

All drivers are also modifed to add the appropriate caps.

Some drivers were incorrectly using non-Gallium-default conventions,
and caps for them have them set so that they will behave correctly
after the later state tracker patches.

This drivers are softpipe/llvmpipe (uses integer rather than half
integer) and pre-nv50 Nouveau (uses lower left rather than upper left).

Other drivers might be broken. With this patchset, fixing them is
only a matter of exposing the appropriate caps that match the behavior
of the existing code.

Drivers are encouraged to support all conventions themselves for better
performance, and this feature is added to softpipe in a later patch.