avcodec/frame_thread_encoder: Avoid allocations of AVFrames

avcodec/frame_thread_encoder: Avoid allocations of AVFrames

Up until now, when using frame threaded encoding, an AVFrame would be
allocated for every frame to be encoded. These AVFrames would reach the
worker threads via a FIFO of tasks, a structure which contained the
AVFrame as well as an index into an array which gives the place where
the worker thread shall put the returned packet; in addition to that,
said structure also contained several unused fields.

This commit changes this: The AVFrames are now allocated during init in
the array that is up until now only used to return the packets. The
contents to be encoded are put into the AVFrame in the same array
element that is also used to return the packets.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>