gallivm: fix sampling with texture offsets in SoA path

gallivm: fix sampling with texture offsets in SoA path

When using nearest filtering and clamp / clamp to edge wrapping results could
be wrong for negative offsets. Fix this by adding the offset before doing
the conversion to int coords (could also use floor instead of trunc int
conversion but probably more complex on "typical" cpu).

This fixes the piglit texwrap offset failures with this filter/wrap combo
(which only leaves the linear/mirror repeat combination broken).

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>