gallivm: simplify sampler interface

gallivm: simplify sampler interface

This has got a bit out of control with more and more parameters added.
Worse, whenever something in there changes all callees have to be updated
for that, even though they don't really do much with any parameter in there
except pass it on to the actual sampling function.
Hence simply put almost everything into a struct. Also instead of relying
on some arguments being NULL, be explicit and set this in a key (which is
just reused for function generation for simplicity). (The code still relies
on them being NULL in the end for now.)
Technically there is a minimal functional change here for shadow sampling:
if shadow sampling is done is now determined explicitly by the texture
function (either sample_c or the gl-style tex func inherit this from target)
instead of the static texture state. These two should always match, however.
Otherwise, it should generate all the same code.

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