Implement AutoFit option in mpv

[Cpuroast]

Setting the following option would start video playback with a window size of 1 to 100% of the display's resolution, scaling up or scaling down as needed taking into consideration of the video's aspect ratio and never display letter-boxing.

-autofit=75% or 0.75

Setting the following option would behave the same, however it would do so only if the video resolution is larger than the display's resolution, therefore it would only need to scale down and never scale up.

-autofit-larger=75% or 0.75

Added as 7885fce.

Note that --autofit=75% means 75% of the width, while --autofit=75%x75% means 75% of either direction.