llvmpipe: Implement TXQ.

llvmpipe: Implement TXQ.

Piglits test for fragment shaders pass, vertex shaders fail.  The
actual failure seems to be in the interpolators, and not the
textureSize query.

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: José Fonseca <jose.r.fonseca@gmail.com>