Support rendering offset about vhs

[griimick]

@maaslalani We can use the -start_number option to set the starting frame in ffmpeg image2 and move the frames which are before it to end of the sequence by renaming their file names.

I would like to take a crack at this if you agree.

[maaslalani]

To start we can just do # of frames, if that's easier for you!

[maaslalani]

Proposed syntax:

# Take the first 10% of the video and place it after the end.
Set LoopOffset 10%
# Take the first 10 frames and place them at the end.
# The first frame of the video will be the 11th frame.
Set LoopOffset 10

[muesli]

Sounds great!

[maaslalani]

@griimick Sounds perfect to me!