i965/gen4: Fix fragment program rectangle texture shadow compares.

i965/gen4: Fix fragment program rectangle texture shadow compares.

The rescale_texcoord(), if it does something, will return just the
GLSL-sized coordinate, leaving out the 3rd and 4th components where we
were storing our projected shadow compare and the texture projector.
Deref the shadow compare before using the shared rescale-the-coordinate
code to fix the problem.

Fixes piglit tex-shadow2drect.shader_test and txp-shadow2drect.shader_test

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69525
NOTE: This is a candidate for stable branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>