This is a follow-up to #287, specifically about storage buffers.
In particular, the following questions need to be answered:

1. given a buffer that's used as read-only storage in WGSL, is it compatible with a GPUBindGroupLayoutEntry that is read-write? I don't think the spec has clear rules for this.
2. are multiple read-write storage bindings be compatible within the same usage scope? Currently, this is allowed by the spec.
3. is read-only storage usage compatible with read-write storage usage? Currently the spec says no.