(WIP) colorspace: implement Dolby Vision Profile 5 (IPT-PQ)

Documentation taken primarily from Patent WO 2016/172091 A1.

Some values subject to change. Needs proper testing. Still looks a bit
funny, but we're heading in the right direction. (I noticed a blue shift
in the files available to me, and stuff is also generally a bit darker
than the equivalent HDR10 files).