Maintain an output queue of AVFrames instead of MediaBuffers
so that the latter can be released early. This avoids a potential deadlock
between the stagefright decoder::read() and Stagefright_decode_frame()
Comment-by-michael: iam commiting this as the code cannot work without it and likely works with it.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>