Introduce patrol cycle to make 'loop' animations

[kabachuha]

The best place to add its controls is the 'Video output' tab

[ryango]

I've taken a hack at this on my own a couple times with pretty decent results, but I usually end up basically mixing frame 0->40 with (max_f-40) -> (max_f) and then stitch_from_frame 40. It works but you end up missing that nice txt2img frame (and I imagine it doesn't work with all the different modes). This is the gist of the change. Would obviously do it proper to make a PR.

Would the idea here be that you run deforum backwards from 0 with inverted transforms x times and then mix in frame -X->0 with (max_f - X) -> max_f? Or is patrol cycle a specific term I'm not familiar with? I would definitely be interested in picking this up as a first issue.