glsl/linker: fix varying packing for non-flat integer varyings.

Commit dfb57e7 (glsl: Fix error checking on "flat" keyword to match
GLSL ES 3.00, GLSL 1.50) relaxed the rules for integral varyings: they
only need to be declared as "flat" if they are a fragment shader
inputs.  This allowed for the possibility of a vertex shader output
being a non-flat integer, provided that it was not matched to a
fragment shader input.  A non-contrived situation where this might
arise is if a vertex shader generates some integral outputs which are
consumed by tranform feedback, but not by the fragment shader.

Unfortunately, lower_packed_varyings assumes that *all* integral
varyings are flat, regardless of whether they are consumed by the
fragment shader.  As a result, attempting to create a non-flat
integral vertex output of a size that required packing (i.e. a size
other than ivec4 or uvec4) would cause an assertion failure in
lower_packed_varyings.

This patch prevents the assertion failure by forcing vertex shader
outputs to be "flat" whenever they are not consumed by the fragment
shader.  This should have no effect on rendering since the "flat"
keyword only affects the behaviour of fragment shader inputs.

Fixes piglit test "spec/EXT_transform_feedback/nonflat-integral".

NOTE: This is a candidate for the 9.1 release branch.
glsl: Document lower_packed_varyings' "flat" requirement with an assert.

To minimize the variety of type conversions that lower_packed_varyings
needs to perform, it assumes that integral varyings are always
qualified as "flat".  link_varyings.cpp takes care of ensuring that
this is the case (even in the circumstances where GLSL doesn't require
it).

This patch documents the assumption with an assertion, for ease in
future debugging.
glsl/linker: Adapt flat varying handling in preparation for geometry shaders.

When a varying is consumed by transform feedback, but is not used by
the fragment shader, assign_varying_locations() sets its interpolation
type to "flat" in order to ensure that lower_packed_varyings never has
to deal with non-flat integral varyings (the GLSL spec doesn't require
integral vertex outputs to be flat if they aren't consumed by the
fragment shader).

A similar situation will arise when geometry shader support is added,
since the GLSL spec only requires integral vertex shader outputs to be
flat when they are consumed by the geometry shader.  This patch
modifies the linker to handle this situation too.