vk/compiler: mark inputs/outputs as read/written
vk/compiler: mark inputs/outputs as read/written

This doesn't handle inputs and outputs larger than a vec4, but we plan
to add a varyiing splitting/packing pass to handle those anyways.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler