mmal/vout: Properly populate plane specification

mmal/vout: Properly populate plane specification

The pictures plane specifications were always filled with the size information
obtained from the vlc picture format combined with the assumption of having 3
planes. As this is not correct for the opaque case were only one plane with a
fixed size, which matches the mmal video_render input size, is used, change
the code to take care of this.
This change is required to provide other plugins reliable information about
the allocated data buffer size.

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