Seek backwards/forwards in replays by checkpoint

[JoeOsborn]

This adds three keybinds/text commands: make a checkpoint, go to previous checkpoint, and go to next checkpoint. It also adds a text (stdin/socket) command to seek a replay to a specific frame, which will also return (via the command replier) the frame which was actually seeked-to (for now, it will only seek to checkpoint frames, since afaik there is no API for running retroarch forward by some number of frames).

This even works for replays being recorded; backwards seeking works as expected but forwards seeking can also work if the replay was seeked-backwards while paused.

I would like particularly close review around:

• adding the keybinds, since it touches many files
• the single-frame unpause (with run_frames_and_pause) used to activate the forward/back scanning logic while paused
• refactoring the movie "reset to beginning of playback" code

I like to bind "make checkpoint" to f2 (same as save-state) and use n/m to go back/forward through checkpoints.

In the future it would be good if seeking backwards could use the backref field in replay frames to more efficiently go backwards, but it would depend on remembering the file position at which a statestream block was written in the replay file so it could be reloaded if it had been garbage-collected (which feels like a separate PR). For now, scanning backwards requires scanning forwards from the beginning of the replay, which is not ideal but it is what it is.

[Jamiras]

How are checkpoints created automatically?

[JoeOsborn]

There's an interval in seconds that's a setting in the RA config.

[JoeOsborn]

I want to change the "find the previous checkpoint" code to not be side-effecting on the movie, so please don't merge until my next patch lands. I also am adding the "seek forward or back to (the closest replay checkpoint before) a given frame number" text command.