Retain rotation metadata intact

[mallniya]

I've noticed some strange behavior: I took an mp4 with rotation of 180 degrees in metadata, cut it with the editor and had an output video with an absent rotation metadata. I guess it's almost always one wants to retain rotation metadata. Also there could be an option to set metadata rotation (or other metadata flags perhaps) by hand.

I know there is already a duplicate of rotation feature request https://github.com/ozmartian/vidcutter/issues/179, though there was no mention of metadata even in original https://github.com/ozmartian/vidcutter/issues/52 and with mobile phones' videos there could be even no problems if source metadata just don't be ignored.