Implement proper scaling in getScaledFrameAtTime()

[3c71]

When requesting a frame, we don't necessarily know it's size in advance, hence ratio is unknown.

On stock Android MediaMetadataRetriever, passing the desired size as a square doesn't affect the resulting ratio of the thumbnail, it is properly scaled to fit within the desired size.

I tested in release 1.0.14, and I end-up with distorted thumbnails, mostly square while they are 16:9 or 21:9.