glsl: Checks for interpolation into its own function.
glsl: Checks for interpolation into its own function.

This generalizes the validation also to be done for variables inside
interface blocks, which, for some cases, was missing.

For a discussion about the additional validation cases included see
https://lists.freedesktop.org/archives/mesa-dev/2016-March/109117.html
and Khronos bug #15671.

Reviewed-by: Samuel Iglesias Gonsálvez <b753b212f30aed827c3eea2e437609dc37310153@igalia.com>
Signed-off-by: Andres Gomez <1948e91ee7852f22bc880d172a32ffca98fb60fa@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler