Normally, we would have to check the framebuffer, but we don't know its
contents until draw time. To avoid yet another situation where we must
compile multiple pipelines, I've used a simple heuristic: if the vertex
pipeline writes to `BuiltInLayer`, this is likely for a layered
framebuffer, and we should use `texture_2darray` for subpass input.
Hopefully this is good enough for all intents and purposes. If not, then
we really will have to wait until draw time. And God help us if someone
try to do this with a 3D texture!
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