asahi: Implement color masks with masked stores

Blend states can require masking colour. Currently, this is handled by nir_lower_blend, which lowers masks to a read-modify-write operation as required on Mali hardware. However, our "tilebuffer store" instruction supports a write mask, allowing us to write only a subset of channels to the tilebuffer. It's more efficient to use that than to emit pointless tilebuffer loads.

Note that even without tilebuffer loads, non-opaque masks don't work with opaque pass types. Here, we handle this with a translucent pass type, which gets HSR to do the right thing and is consistent with the pass type used previously. However, it's a bit heavy handed -- Apple manages to use an opaque pass type with masking but with some unknown HSR fields twiddled. IMO reverse-engineering those details shouldn't block this because this gets us closer to optimal (just not all the way there) and is strictly better than what we had before.