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Implement experimental GPU two-phase occlusion culling for the standard
3D mesh pipeline. *Occlusion culling* allows the GPU to skip the vertex and fragment shading overhead for objects that can be quickly proved to be invisible because they're behind other geometry. A depth prepass already eliminates most fragment shading overhead for occluded objects, but the vertex shading overhead, as well as the cost of testing and rejecting fragments against the Z-buffer, is presently unavoidable for standard meshes. We currently perform occlusion culling only for meshlets. But other meshes, such as skinned meshes, can benefit from occlusion culling too in order to avoid the transform and skinning overhead for unseen meshes. This commit adapts the same [*two-phase occlusion culling*] technique that meshlets use to Bevy's standard 3D mesh pipeline when the new `OcclusionCulling` component, as well as the `DepthPrepass` component, are present on the camera. It has these steps: 1. *Early depth prepass*: We use the hierarchical Z-buffer from the previous frame to cull meshes for the initial depth prepass, effectively rendering only the meshes that were visible in the last frame. 2. *Early depth downsample*: We downsample the depth buffer to create another hierarchical Z-buffer, this time with the current view transform. 3. *Late depth prepass*: We use the new hierarchical Z-buffer to test all meshes that weren't rendered in the early depth prepass. Any meshes that pass this check are rendered. 4. *Late depth downsample*: Again, we downsample the depth buffer to create a hierarchical Z-buffer in preparation for the early depth prepass of the next frame. This step is done after all the rendering, in order to account for custom phase items that might write to the depth buffer. Note that this patch has no effect on the per-mesh CPU overhead for occluded objects, which remains high for a GPU-driven renderer due to the lack of `cold-specialization` and retained bins. If `cold-specialization` and retained bins weren't on the horizon, then a more traditional approach like potentially visible sets (PVS) or low-res CPU rendering would probably be more efficient than the GPU-driven approach that this patch implements for most scenes. However, at this point the amount of effort required to implement a PVS baking tool or a low-res CPU renderer would probably be greater than landing `cold-specialization` and retained bins, and the GPU driven approach is the more modern one anyway. It does mean that the performance improvements from occlusion culling as implemented in this patch *today* are likely to be limited, because of the high CPU overhead for occluded meshes. Note also that this patch currently doesn't implement occlusion culling for 2D objects or shadow maps. Those can be addressed in a follow-up. Additionally, note that the techniques in this patch require compute shaders, which excludes support for WebGL 2. 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. [*two-phase occlusion culling*]: https://medium.com/@mil_kru/two-pass-occlusion-culling-4100edcad501 [Aaltonen SIGGRAPH 2015]: https://www.advances.realtimerendering.com/s2015/aaltonenhaar_siggraph2015_combined_final_footer_220dpi.pdf [Some literature]: https://gist.github.com/reduz/c5769d0e705d8ab7ac187d63be0099b5?permalink_comment_id=5040452#gistcomment-5040452 [SPD-based hi-Z buffer shader from the Granite engine]: https://github.com/Themaister/Granite/blob/master/assets/shaders/post/hiz.comp
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