handle scaling of vertical video

[asymmetricia]

Hi!

I have a vertical cellphone video that's rotated via what ffmpeg registers as stream_side_data rotation.

Without any of these changes, discord_video.sh deforms the video to fit in 1280x720; ffmpeg automatically rotates the input, which then has resolution 1080x1920. The -vf scale stretches the width to 1280 while shrinking the height to 720.

This change does two things:

1. Compute the -vf scale according to whichever of width or height is greater, and lets ffmpeg retain the aspect ratio automatically by setting the other element to -1
2. As part of this, also check the stream_side_data rotation to understand when ffmpeg will internally flip width & height.

[Czechball]

Hey, thanks for the improvement! Everything seems great, I never thought there could be issues with vertical videos... @HUNcamper could you please take a look at the Batch logic for this? My Windows scripting still sucks :(

[selimnahimi]

Will do 👌