Fix for poster image aspect ratio and issue with shrinking of larger poster images

[JesseHC]

This is related to Aspect ratio not being honored for video player sidecar poster images, but I figured you would want me to post it here and not mix things up.

I know you are planning on getting rid of mediaelementjs_player, but if you change line 112 in mediaelementplayer.css from:
background-size: cover;
to:
background-size: contain;

It will properly scale the image posters in that as well to their correct aspect ratio in the video player.

The one issue I have noticed with mediaelementjs_player and how it handles poster images (and this may relate eventually to Zenphoto as well), if the image is larger than the set poster size, but a different aspect ratio, it will stretch the image when it shrinks it. This is not a problem if the images are smaller, it will keep them at the smaller size, and scale them in the player itself.

So a 1440x1080 sidecar image gets stretched to 1280x720, but a 640x480 image will stay at 640x480.

[acrylian]

Please note: The repo is not set into archive mode because I forgot but - as noted in the readme – it is not developed anymore.

[acrylian]

Btw, the file in question is from the player itself. We don't modify parts of 3rd party scripts generally because that gets overwritten on updates.