i965/vs: Add support for texel offsets.

i965/vs: Add support for texel offsets.

The visit() half computes the values to put in the header based on the
IR and simply stuffs that in the vec4_instruction; the emit() half uses
this to set up the message header.  This works out well since emit() can
use brw_reg directly and access individual DWords without kludgery.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>