asahi: Add new clear/reload/store infrastructure

asahi: Add new clear/reload/store infrastructure

With multiple render targets, it's not practical to generate all
variants of the background and end-of-tile programs at start up. Rather
than trying, add a hash table of meta program keys to background
programs, and compile variants as they're needed.

With the new infrastructure, it's sensible to handle clears with the
same code path as reloads. In addition to getting us closer to multiple
render target support, this gets us support for non-RGBA8 render
targets, as the u8norm tilebuffer format was baked into the hardcoded
clear shader and store shaders used before.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19871>