transcode: replace PICTURE_RING_BUFFER with picture_fifo_t

transcode: replace PICTURE_RING_BUFFER with picture_fifo_t

The PICTURE_RING_BUFFER used a fixed size, which when running out of available pictures
produces visible artefacts in the encoded stream. Allocating a new picture and keeping
track of them in a fifo is much simpler (in the multiple threads encoding scenario) and
solves the ring buffer overrun.