softpipe: rework texture sampling code

softpipe: rework texture sampling code

Split into component pieces, stitch together at runtime using function
pointers.  Make it possible to utilize the existing fastpaths as image-level
filters for generic mip-filtering routines.

Remove special case for rectangle filtering, as it can now be handled by
the 2d path.

As most of the mesa demo texturing was already covered by fast paths, its
harder to find examples of speedups, but tunnel gets a boost as mip-nearest
filtering is now able to access the img_2d_linear_wrap_POT functions
for sampling within a mipmap level.