i965 Gen4/5: clip: Add support for noperspective varyings

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.

V5: Add brw_clip_compile.has_noperspective_shading rather than another
key flag.
V6: Real bools.

[V1-2]: Signed-off-by: Olivier Galibert <galibert at pobox.com>
Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>