Allows an application to register a callback that manages mutexes
on behalf of FFmpeg.
With this callback registered FFmpeg is fully thread safe.

backport r19025 by andoma

NB: This is a feature backport with little regression potential. It was
requested at FOSDEM 2010 by ben@geexbox.org for use by geexbox and the
enna mediacenter in the upcoming debian/squeeze and ubuntu/lucid
release.

Approved by DonDiego on #ffmpeg-devel


Originally committed as revision 21731 to svn://svn.ffmpeg.org/ffmpeg/branches/0.5

Originally committed as revision 17697 to svn://svn.ffmpeg.org/ffmpeg/trunk