* use WeakHybrid in InternalRenderlet

* Occlusion culling, part one

added MipMapGenerator, depth pyramid shaders

slab buffers are created with size 1 to avoid wgpu validation bug

fixed a bug where re-staging a texture often copied to the wrong layer

debbugging led to better sphere projection for culling

update sample count so we don't invalidate every frame

debug overlay of bounding spheres

use box instead of circle for bounds in debug overlay

don't use compute culling by default

* regenerate shaders, fix merge errors, treat *.spv and *.wgsl as binary

* use cargo-gpu on branch

* clean rust-gpu checkouts (temporary)

* also clean the rust-gpu cache dir

* allow the cache to persist

* don't use bool as u32 in shaders

* change rust-gpu cache step

* update cargo gpu