Fix scrubber sprite creation for short video files

[bnkai]

As it is now stash skips creation of the scrabber sprites for very short video files (< 3s)
This PR enables generation of sprites for files with low duration/frame count

For files that are < 5s or the frame count is smaller than the chunkcount (9x9) it uses a slower method to generate them
For every thumbnail extraction we use the select ffmpeg filter to extract a single frame (slow and should only be used for small files) from the video
If the number of frames are smaller than needed then duplicate frames are added to compensate