nvfx: refactor sampling code, add support for swizzles and depth tex

nvfx: refactor sampling code, add support for swizzles and depth tex

This is a significant refactoring of the sampling code that:
- Moves all generic functions in nvfx_fragtex.c
- Adds a driver-specific sampler view structure and uses it to
  precompute texture setup as it should be done
- Unifies a bit more of code between nv30 and nv40
- Adds support for sampler view swizzles
- Support for specifying as sampler view format different from the
  resource one (only trivially)
- Support for sampler view specification of first and last level
- Support for depth textures on nv30, both for reading depth and
  for compare
- Support for sRGB textures
- Unifies the format table between nv30 and nv40
- Expands the format table to include essentially all supportable formats
  except mixed sign and "autonormal" formats
- Fixes the "is format supported" logic, which was quite broken, and
  makes it use the format table

Only tested on nv30 currently.