Compute passes with shared usage scope

[fintelia]

Is your feature request related to a problem? Please describe.
Right now, wgpu automatically inserts barriers between any two compute shader dispatches that write to the same buffer. This is necessary in the general case because the programmer may be relying on the dispatches happening sequentially. However, if it is safe to run the dispatches in parallel (say because they write to different regions of the buffer, or use atomics to avoid conflicting with one another) then this can be very wasteful.

Describe the solution you'd like
A field on ComputePassDescriptor to indicate whether the whole pass should be one usage scope, instead of the current statregy of a usage scope per dispatch.

Describe alternatives you've considered
The user can accept reduced performance or devise potentially complicated schemes to merge all their dispatches into one.

[cwfitzgerald]

For information:

We will likely implement this using a member of the compute pass descriptor:

enum ComputePassType {
    // One usage scope per dispatch
    Serial,
    // One usage scope per pass
    Concurrent,
}

This would manifest as wgpu keeping track of a compute pass as a single usage scope. On metal we would supply https://developer.apple.com/documentation/metal/mtldispatchtype to the compute pass arugments.