Partially bound descriptors on Metal

[kanerogers]

Background

This depends on #3334 and requires some modifications to naga to emit the correct shader configuration.

Further reading

https://developer.apple.com/documentation/metal/buffers/managing_groups_of_resources_with_argument_buffers

[John-Nagle]

The main problem with bindless seems to be who maintains the big descriptor table. That has to be allocated and kept in sync, on an element by element basis, with the texture buffers.

Vulkan expects its caller to maintain the descriptor table.
Vulkano, a safe Rust interface to Vulkan, maintains the descriptor table itself. To achieve safety, the descriptor table has to be maintained inside the safety boundary, I think. So WGPU will need to do that to be safe.

From the discussion above, it looks like this can work fine for Vulkan, Metal, and DX12. For WebGPU, there is apparently extra copying involved. Android seems to have bindless now. OpenGL has a bindless extension, but I don't know how broad support is.

It looks like all targets now support bindless mode in current versions. The main question is how much legacy system support is desired.