glsl: split layout_defaults into specific types
glsl: split layout_defaults into specific types

This will allow merging of duplicate layout qualifiers as allowed
by ARB_shading_language_420pack

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Chris Forbes <5dcac483e7a6e31a5cbe89f38cc69dff8ee93dac@ijw.co.nz>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler