nir: move to two-level binding model for UBO's

nir: move to two-level binding model for UBO's

The GLSL layer above is still hacky, so we're really just moving the
hack into GLSL-to-NIR. I'd rather not go all the way and make GLSL
support the Vulkan binding model too, since presumably we'll be
switching to SPIR-V exclusively, and so working on proper GLSL support
will be a waste of time. For now, doing this keeps it working as we add
SPIR-V->NIR support though.