Swap to a hi-Z buffer approach for occlusion culling and make it non-experimental #18711
Description
This PR is marked experimental because of known precision issues with the downsampling approach when applied to non-power-of-two framebuffer sizes (i.e. most of them). These precision issues can, in rare cases, cause objects to be judged occluded that in fact are not. (I've never seen this in practice, but I know it's possible; it tends to be likelier to happen with small meshes.) As a follow-up to this patch, we desire to switch to the SPD-based hi-Z buffer shader from the Granite engine, which doesn't suffer from these problems, at which point we should be able to graduate this feature from experimental status. I opted not to include that rewrite in this patch for two reasons: (1) @JMS55 is planning on doing the rewrite to coincide with the new availability of image atomic operations in Naga; (2) to reduce the scope of this patch.
WGPU 24 has landed in #17542, so I believe this is now unblocked.