Expose readonly size/usage on wgpu::Buffer

[swiftcoder]

Is your feature request related to a problem? Please describe.

Per the draft spec, it appears that GPUBuffer is supposed to expose it's size and usage via readonly properties:
https://gpuweb.github.io/gpuweb/#buffer-interface

As of wgpu-rs 0.13.1, on the rust side neither of these properties are currently visible to application code. Also the wgpu::Buffer type doesn't appear to be caching the size at all.

Describe the solution you'd like

Keep a cached copy of the size on wgpu::Buffer, and expose member functions to provide readonly access to both the size and usage fields.

Describe alternatives you've considered

The application can always track these attributes externally, since they are immutable after construction, but I'd hate to permanently incur that added burden on application code.

[kpreid]

I've heard that WebGPU and/or wgpu intentionally don't provide “getters” for most state, so that the data flow is primarily CPU-to-GPU without ever pausing to fetch data back or requiring wgpu to speculatively retain information in case it is needed.

However, even not considering that the spec does specify to expose buffer sizes, they would be especially useful, because whenever a buffer has dynamically sized contents, it may be useful to understand the buffer's size as a “capacity” (in the sense of Vec::capacity()), potentially use less than all of the buffer, and consult the size to determine whether allocating a new buffer is necessary.

[swiftcoder]

and consult the size to determine whether allocating a new buffer is necessary.

Yep, that's exactly the case I'm running into. I have an unpredictable number of mesh instances, and the instance data buffer sometimes needs to grow to accommodate additional instances