Vulkan allows fragment shaders to read gl_ClipDistance[], in which case
the SPIR-V compiler will inject a compact array variable at
VARYING_SLOT_CLIP_DIST0. Request the lowering to always work in terms
of a compact array variable so we don't have to care about the API
in use.
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6022>
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