gallivm: implement texture border color, plus tweaks to some wrap modes

gallivm: implement texture border color, plus tweaks to some wrap modes

The progs/test/texwrap demo looks pretty good, but there are still some
tiny differences from softpipe.  There may be a sub-pixel texcoord
interpolation error somewhere.

There's some room for optimization.  Many of the wrap modes compute
intermediate values that are constant for the texture size (see the
min/max values).  These could be computed earlier and stored somewhere
for later use.