mmal/vout: Make display() synchronous

mmal/vout: Make display() synchronous

Ensure that the pictures which are rendered by display() are released before
display() returns. This is required to ensure that on stop all pictures are
released and the core can safely cleanup the pool. Without this the core
might detect pictures as leaks and free them on its own. Once this happens
we're running straight into a double free.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>