API for specifying buffer and texture wgpu usage

[almarklein]

Specific use-cases, perhaps using custom shaders, may require buffers or textures to have additional usage flags, see e.g. #567. There is resource._wgpu_usage, which is a private flag intended for internal usage. We could expose it, but that would add wgpu- stuff into our main API, which we're trying to avoid ... 🤔 (there's a few more places where this happens)

xref #272