Remove defunct option for building without FFmpeg

I tried to compile OpenMW for the first time, but I forgot to install ffmpeg-devel (on Fedora). This led to the build failing.

However, looking at the code it seems like the client should also run without ffmpeg. For example in mwrender/videoplayer.cpp there are two definitions for VideoState, one is to be used without ffmpeg, but it misses mTexture, so the build won't compile. mwsound/soundmanagerimp.cpp also doesn't handle SOUND_IN being undefined.

Fixing those simple issues leaves a runtime error where an exception is thrown in the destructor of OpenAL_Sound. I just added a catch() around it as a quick workaround and the game worked without ffmpeg.

Since it seems like nobody runs the client without FFmpeg I'm not sure whether you want to fix this or disallow running without FFmpeg.

(RM-1716 from redmine: created on 2014-07-28 by George Host, , closed on 2014-07-28 by George Host)