Identify common codec support in ffmpeg across all platforms

[lnihlen]

Recent work on #11 points out that some of the common codecs are missing in the current windows build of ffmpeg.

Important to build a set of minimum codecs for decode and encode that ffmpeg can be expected to have, for still images, video, and audio. Then folks can rely on a certain base set of codecs to always be present.

Might be worth establishing a support matrix for the codecs, identify holes with issue numbers, and build a media test suite to ensure coverage.