Adding mastering display metadata struct to avutil. The mastering display metadata contains information
about the mastering display color volume (SMPTE 2086:2014).

This info comes from HEVC in the SEI_TYPE_MASTERING_DISPLAY_INFO and is soon to be included in MKV:
https://mailarchive.ietf.org/arch/search/?email_list=cellar&gbt=1&index=sZyfPTM-QY69P-0omfOIiTN622o
so it is similar to SEI FPA / stereo_mode in MKV and as such this patch follows how AVStereo3D is implemented.

I'll add support to HEVC in a follow-up (and MKV when spec is approved).
Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>