softpipe: implement TXF support via get_texel callback

softpipe: implement TXF support via get_texel callback

This just calls the texel fetch functions directly bypassing the sampling,

notes:
1: loops inside switch should be more optimal.
2: borders can be sampled though only up to border depth, outside that
its undefined.

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