gstdecode: handle fourcc mismatch between gst vlc

Add raw video fourcc conversion support to convert
from gstreamer representation (for strings that are
not equal to length 4) to vlc representation.

Signed-off-by: Thomas Guillem <thomas@gllm.fr>
(cherry picked from commit a6c52d89)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>