tgsi: add support for texture offsets to the TGSI IR. (v2)

tgsi: add support for texture offsets to the TGSI IR. (v2)

This adds tokens for texture offsets, to store 4 * swizzled vec 3
for use in TXF and other opcodes.

It also contains TGSI exec changes for softpipe to use this code,
along with GLSL->TGSI support for TXF.

v2: add some more comments, add back padding I removed.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>