mmal: Use zerocopy ports for opaque mode

mmal: Use zerocopy ports for opaque mode

When running in opaque mode the mmal codec, deinterlace and vout must be used
together, so it is known that all pictures in use are allocated by the mmal
vout and contain an associated mmal_buffer_header. This allows us to enable
the zerocopy feature of mmal, which allocates shared memory between GPU and
CPU for holding the buffer payloads. Albeit the payloads are just small opaque
handles that reference the GPU side pictures it saves a little bit of
performance, because less VCHIQ transfers are required.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>