zerocodec: fix direct rendering.

Set picture type before calling get_buffer.
This allows the DR application to make better decisions.
It also fixes a resource leak in case of missing reference frames
since it would call get_buffer but never release_buffer.
Also use FFSWAP to ensure that the AVFrame is properly initialized
in the next get_buffer (in particular that data[0] is NULL).

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
