build: add --disable-ffmpeg flag.

I can already hear the screams... @elupus: I'm prepared for my lashings.

Some who package XBMC may be using hardware decoders and don't want to take on the legal responsibility of shipping ffmpeg. This provides that option. It is meant to go along with PR 911, but split off as this one is just an RFC.

Clearly this is just to show what needs to happen to remove ffmpeg from build.
It is not clean or pretty, and is only meant to start the discussion.
In order to do this cleanly, a few things should happen:

- We need an internal image scaling/resampling class with swscale as one of
  the implementations. That will avoid the ifdefs for swscale all over.

- We need our own representation of codec id's. These could map to ffmpeg's
  if it's available, but stand alone if not.