glsl: store stage reference in gl_uniform_block

glsl: store stage reference in gl_uniform_block

This allows us to simplify the code and drop InterfaceBlockStageIndex
which is a per stage array of integers the size of all blocks in the
program combined including duplicates across stages. Adding a stage
ref per block will use less memory.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>