st/mesa: Gallium support for ARB_fragment_coord_conventions (v4)

st/mesa: Gallium support for ARB_fragment_coord_conventions (v4)

Changes in v4;
- Implemented Brian Paul's style suggestions

Changes in v3:
- Use positive caps instead of negative ones

Changes in v2:
- Updated formatting

The state tracker will use the TGSI convention properties if the hardware
exposes the appropriate capability, and otherwise adjust WPOS itself.

This will also fix some drivers that were previously broken due to their
incorrect, inadvertent, use of conventions other than upper_left+half_integer.