Feature request: Pre pass callbacks.

[zombietype]

Currently, there exist per-resource callbacks for preRead and preWrite.
This allows queueing up transitions etc.

However, it would be useful to have a pre-pass callback that can operate on the context that allows batching the barriers.

Currently, I need to manually call a method to do this in every pass.

There can be arguments for post-pass etc, but I can only find a pre-pass being important at the moment.

[zombietype]

I would like to contribute to this myself. I'll submit a PR soon.