Add a full-codecs extension

Adds a platform extension containing ffmpeg libraries with a good amount of codecs in them, this should prevent so many apps needing to rebuild ffmpeg.

This extension is intended to be used in the same way as the platform compat extensions, that is specified by an application, as in the tests/test.codecs.ffmpeg-full.json manifest. This means that only apps that request the extra codecs actually get them, while the extension is still shared and downloadable independently (or removable independently). This also means that apps can choose whether to have these codecs autodownload or not, removing the responsibility of that decision from us :P

I have confirmed that the codecs are found correctly and can be used (I have tested a .wmv video, at least).

This MR replaces !918 (closed), as that got messy and confused from a lot of changes.

Also rejigs our ffmpeg config into a much easier format to work with, and enables a few more codecs in the base ffmpeg.

Closes #599 (closed)