vkreplay one frame at a time

[KarenGhavam-lunarG]

The current vkreplay interface lets you hit the space key to pause replay and hit it again to resume. There needs to be a way to replay a frame at a time. This seems like a very usable feature.

This came in the latest LunarG ecosystem survey: Ability to single step through each frame. I've used it several times to try to catch issues that occur on a single frame, ended up hacking in something so that I can stop at a specific frame and then capture that with RenderDoc.