[Feature] Flatten VR preview videos for scenes and markers

[vamiceDev]

Is your feature request related to a problem? Please describe. Currently, when Stash generates preview videos for VR scenes, or markers from VR scenes, the preview video is in the original VR format, often side-by-side fisheye video. In the scene/marker wall, this is very hard to understand, as it effectively turns the preview video into two, smaller videos.

Describe the solution you'd like An option where videos marked with the appropriate VR Tag generate flat, 2D preview videos instead. This would likely require multiple VR tags for different VR encoding types so that the output is correct.

Describe alternatives you've considered Since generating previews is built into Stash, it might be nice to have this as a feature of Stash itself, but it could also work as a plugin. (If there's a plugin that does this, I don't know where it is.)

Additional context I did some experiments with ffmpeg, and I was able to find some options to produce flat video from 180 side-by-side VR content.

This uses the v360 filter built into ffmpeg: https://ffmpeg.org/ffmpeg-filters.html#v360

Adding -vf v360=in_stereo=sbs:e to the ffmpeg options produces something similar to just one of the two side-by-side channels.

Adding -vf v360=in_stereo=sbs:rectilinear to the ffmpeg options produces a view of the center of the frame.