glsl: Document lower_packed_varyings' "flat" requirement with an assert.

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.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>