Add experimental support for vp9 in iso-bmff
Implemented according to the draft specification
"VP Codec ISO Media File Format Binding":
http://www.webmproject.org/vp9/#draft-vp-codec-iso-media-file-format-binding
'-strict -2' is required to use this feature.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>