i965/vs: Store texturing results into a vec4 temporary.

i965/vs: Store texturing results into a vec4 temporary.

The sampler appears to ignore writemasks (even when correcting the
WRITEMASK_XYZW in brw_vec4_emit.cpp to the proper writemask) and just
always writes all four values.

To cope with this, just texture into a temporary, then MOV out into a
register that has the proper number of components.

NOTE: This is a candidate for stable branches.

Fixes es3conform's shadow_execution_vert.test.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <idr@freedesktop.org>