Supporting Dawn as a backend

[almarklein]

Dawn is Google's implementation of webgpu.h. This (in theory) means that we can use the same ffi to load the dawn library. Though we now also use some wgpu-native-specific stuff via wgpu.h.

At the time of writing there are (that I know) no readily available pre-build binaries. I also don't have a clear incentive for spending time on this right now, but at some point it may get interesting.

Related issues:

• wgpu: webgpu.h backend gfx-rs/wgpu-native#343

[willemkokke]

I came across this in my research: https://machengine.org/pkg/mach-gpu/

They use zig's excellent c/c++ support and build system to cross compile dawn for many plaforms and state this as a future goal:

• Expose a standard Dawn webgpu.h-compliant C ABI, which routes through Zig comptime interfaces.

In which case that would become an excellent source of prebuilt binaries.