asahi: Introduce concept of spilled render targets

asahi: Introduce concept of spilled render targets

To accommodate framebuffers which exceed tilebuffer limits, we'll need to spill
render targets to main memory. In effect, we need to emulate an immediate-mode
renderer for some render targets. This decision is made on a per-render target
basis. In our tilebuffer layout calculation, rather than asserting that all
render targets fit, introduce a notion of spilling.

This doesn't actually implement spilling -- it just pushes the assert failure
down to the users. But it's progress.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24258>