Noticed while testing ogv.js's WebM playback mode:
this 360p transcode should be 640 pixels wide but sets a display size of 641 pixels wide -- due to the ffmpeg video scaler's attempt to preserve the _exact_ aspect ratio of the 3840x1714 original when we request a resize to 640x286 pixels tall. The resulting file encodes 640x286 pixels with a stretched display to 641x286. Would rather it just keep the size we give, knowing slight rounding errors are possible because we set it to an even number.
Can use -vf scale instead of -s and add setsar=1:1 I think, testing.