i965: Add support for gl_SkipComponents[1234].

i965: Add support for gl_SkipComponents[1234].

ARB_transform_feedback3 allows applications to insert blank space
between interleaved varyings by adding fake 1, 2, 3, or 4-component
varyings named gl_SkipComponents[1234].

Mesa's core data structures don't explicitly track these, instead simply
tracking the buffer offset for each real varying.  If there is padding
due to gl_SkipComponents, these will not be contiguous.

Our hardware takes the specification quite literally.  Instead of
specifying offsets for each varying, it assumes they're all contiguous
and requires you to program fake varyings for each "hole".

This patch adds support for emitting SO_DECL structures for these holes.
Although we've lost the information about exactly how the application
specified their padding (i.e. gl_SkipComponents2, gl_SkipComponents2
vs. a single gl_SkipComponents4), it shouldn't matter.  We just need to
emit the right amount of space.  This patch emits the minimal number of
hole SO_DECL structures.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>