[AD] (temporarily) removing the ffmpeg backend from the video addon

[SiegeLord <siegelordex@xxxxxxxxxx>]

So I spent the past 4 days trying to fix the various issues with the 
ffmpeg backend (firstly, it doesn't compile due to being so out of date, 
but even if that's fixed, it produces static for most videos due to 
broken audio handling, seeking doesn't work, pausing doesn't work, etc.) 
and I came to the conclusion that the best solution is to just throw it 
away and rewrite the code in its entirety.

My plan for the rewrite is actually to move most of the theora code 
(which works a lot better and is a lot simpler to understand than 
ffmpeg's) into the generic video code, and then implement the ffmpeg 
support on top of that, so the complicated seeking and synchronization 
code is shared between the two backends.

That will take time, and I meanwhile I don't want to spend more time 
fixing the current broken code. Leaving it in as is also seems like a 
bad idea. So I propose just completely removing it for now, with the 
intention to add it back in using the strategy outlined above. I do want 
to add it back in, and then seriously look into packaging it on Windows, 
as I think theora is a terrible video format and ffmpeg would be of 
great use. I can't imagine this will inconvenience anyone, as the 
current code is just broken.

Any opinions on this?