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.