For merging shader stages, it will be useful to express a load from an explicit
GL "descriptor set", so we can represent things like UBO loads with merged
shaders where UBOs can come from either stage. To do so, we add an intrinsic
representing a load from the driver's uniform tables, indexed like "descriptor
sets" with "bindings".
In principle, a layered GL-on-Vulkan implementation would use literal descriptor
sets for each stage, so I feel comfortable with the analogy here.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24847>
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