i965 Gen4/5: Generalize SF interpolation setup for GLSL1.3

Previously the SF only handled the builtin color varying specially.
This patch generalizes that support to cover user-defined varyings,
driven by the interpolation mode array set up alongside the VUE map.

Based on the following patches from Olivier Galibert:
- http://lists.freedesktop.org/archives/mesa-dev/2012-July/024335.html
- http://lists.freedesktop.org/archives/mesa-dev/2012-July/024339.html

With this patch, all the GLSL 1.3 interpolation tests that do not clip
(spec/glsl-1.30/execution/interpolation/*-none.shader_test) pass.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965 Gen4/5: clip: correctly handle flat varyings

Previously we only gave special treatment to the builtin color varyings.
This patch adds support for arbitrary flat-shaded varyings, which is
required for GLSL 1.30.

Based on Olivier Galibert's patch from last year:
http://lists.freedesktop.org/archives/mesa-dev/2012-July/024340.html

[V1-2]: Signed-off-by: Olivier Galibert <galibert at pobox.com>
Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965 Gen4/5: clip: Add support for noperspective varyings

Adds support for interpolating noperspective varyings linearly in screen
space when clipping.

Based on Olivier Galibert's patch from last year:
http://lists.freedesktop.org/archives/mesa-dev/2012-July/024341.html

At this point all -fixed and -vertex interpolation tests work.

[V1-2]: Signed-off-by: Olivier Galibert <galibert at pobox.com>
Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965 Gen4/5: clip: Don't mangle flat varyings

This patch ensures that integers will pass through unscathed.  Doing
(useless) computations on them is risky, especially when their bit
patterns correspond to values like inf or nan.

[V1-2]: Signed-off-by: Olivier Galibert <galibert at pobox.com>
Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>