Since Metal doesn't pass clip distance into the fragment shader, we have to
do it ourselves. The CLIP_DIST0/1 varying slots are used to represent the
user-defined varyings we use to pass them from vertex to fragment and
a new intrinsic is added to represent the write to the built-in
clip_distance variable. Since the CLIP_DIST0/1 varying slots are not affected
by opt_varyings, there can be potential interface mismatches so the machinery
in msl_iomap.c is refactored to allow them to be output as a series of scalars
rather than vectors.
Reviewed-by: Aitor Camacho <aitor@lunarg.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38839>
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