lavf/matroskadec: Add early support for some of the new colour elements.

Adding early support for a subset of the proposed colour elements
according to the latest version of spec:
https://mailarchive.ietf.org/arch/search/?email_list=cellar&gbt=1&index=hIKLhMdgTMTEwUTeA4ct38h0tmE

I've left out elements for pix_fmt related things as there still
seems to be some discussion around these, and the max_cll/max_fall
are currently not propagated as there is not yet side data for them.

The new elements are exposed under strict experimental mode.

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
