FSO doesn't appear to use this any longer. And even when it was enabled the
requirements made it largely useless anyway. Removing to clean up cfile a
bit and so we don't have to keep maintaining it.
FFmpegContext can now also hold a cursor over a vector of bytes instead of a cfile to store sound data in it.
Appropriate creation, reading seeking functions included.
Invalid values previously corrupted the internal position counter which
caused crashes on some platforms because the position counter would
overflow in the next read operation. I added some assertions to track
the position counter corruption and it's probably a good idea to leave
them in to catch possible future errors early.
The actual fix was to just ensure that the seek position it always valid
for the file.
This fixes#1059.
Taylor Richards