asahi: Write to cubes/etc attachments as 2D array

asahi: Write to cubes/etc attachments as 2D array

To reduce shader variants, the tilebuffer lowering code does not know the
actual texture targets of the spilled render targets, only whether they are
layered or not. As such, all layered targets (3D, cube map, etc) get written out
uniformly as 2D Arrays. For that to work, the driver needs to do the
corresponding transform.

Regular imageStore() instructions are not affected by any of this.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>