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

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.

V5: Move key.do_flat_shading to brw_sf_compile.has_flat_shading; drop
vestigial hunks.
V6: Real bools.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>