softpipe: handle vertex texture sampling when using llvm for draw

softpipe: handle vertex texture sampling when using llvm for draw

Pretty trivial, just fill in the offsets and such. The implementation
is near 100% copy and paste from llvmpipe. Should be useful for debugging.

No piglit change when not using SOFTPIPE_USE_LLVM=1.
Now that it can do the same tests with and without using llvm for vs/gs,
with llvm more pass, the only things failing only with llvm seems to be
edgeflags tests and vs/gs-pow-float-float (and for the latter I'm not
convinced the zero tolerance it requires is somehow mandated by glsl).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>