vdec: add support for get_buffer()
vdec: add support for get_buffer()

So that we can align the AVFrames data ourselves, and write directly to
the next element's data.

This should be revised when GStreamer has stride support.

Signed-off-by: Felipe Contreras <de77a8b9cd239b204c19c52ed62e48ab47243362@gmail.com>

felipec/gst-av,felipec/gst-av,felipec/gst-av