Enable zero-copy support for gstreamer video decoders

Implements the interface to provide the VLC pictures
from vout mapped to GstBuffers, along with per video
plane stride/offsets info, to the upstream gstreamer
video decoder. If the stride/offsets info of the vlc
pictures (created by vout) matches the video decoder's
expectations, the decoder will directly decode into
the vout allocated pictures, enabling zero-copy. Else
it will continue to use it's own bufferpool

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>