mpegplayer: Get it in better shape for dual core targets. Utilize the newly added...

mpegplayer: Get it in better shape for dual core targets. Utilize the newly added cache stuff. Add a mutex to core shared buffer variables. I'd prefer to have a true spinlock there but this will do for the moment and protect the data. Nonetheless I can't seem to crash it on an e200 any longer and the display garbage is gone.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@13153 a1c6a512-1295-4272-9138-f99709370657