st/mesa: add texture gather support. (v2)

st/mesa: add texture gather support. (v2)

This adds support for GL_ARB_texture_gather, and one step of
support for GL_ARB_gpu_shader5.

This adds support for passing the TG4 instruction, along
with non-constant texture offsets, and tracking them for the
optimisation passes.

This doesn't support native textureGatherOffsets hw, to do that
you'd need to add a CAP and if set disable the lowering pass,
and bump the MAX offsets to 4, then do the i0,j0 sampling using
those.

Signed-off-by: Dave Airlie <airlied@redhat.com>