Metal doesn't support broadcasting or shuffling boolean values, but we
can work around that by casting it to `ushort`, then casting it back to
`bool`. I used `ushort` instead of `uint` because 16-bit values give
better throughput on Apple GPUs.
If a buffer rewrites its Offsets, all member references to that struct
are invalidated, and must be redirected, do so in to_member_reference,
but there might be other places where this is needed. Fix as required.
SPIR-V code relying on this is somewhat questionable, but seems to be
in-spec.
It'll be useful to have an "auxiliary buffer" for other builtins--e.g.
`DrawIndex` (which should be easier to implement now), or `ViewIndex`
when someone gets around to implementing multiview.
Pass this buffer to leaf functions as well.
Test that we handle this for integer textures as well.
It's intended to be used with MoltenVK to support arbitrary
`VkComponentMapping` settings. The idea is that MoltenVK will pass a
buffer (which it set to some buffer index that isn't being used)
containing packed versions of the `VkComponentMapping` struct, one for
each sampled image.
Yes, this is horribly ugly. It is unfortunately necessary. Much of the
ugliness is to support swizzling gather operations, where we need to
alter the component that the gather operates on--something complicated
by the `gather()` method requiring the passed-in component to be a
constant expression. It doesn't even support swizzling gathers on depth
textures, though I could add that if it turns out we need it.
