Remove fragile WebGL mipmap-based radix sort

Removes the experimental mipmap-prefix-sum GPU radix sort (FramePassRadixSort),
its render passes, the associated GLSL/WGSL shader chunks, and the hidden test
example. The approach relied on a hardware-mipmap prefix-sum trick that is
fragile and fails on many devices, and was never wired into the gsplat
rendering path. The WebGPU compute radix sort and the WebGL CPU worker sort are
unaffected.

Author: Martin Valigursky