videoconvert: minimalist HDR-to-SDR support

While we do have the various primaries and transfer functions to do colorspace conversion between HDR and SDR, the result is wrong.

The problem is that we don't take into account target luminance (of the colorspace) in the conversion. Ex: Max brightness for BT2020 could be 10,000 nits. But for SDR it's generally 100nits.

The result is that when you do a naive video/x-raw,colorimetry="bt2100-pq" ! videoconvert gamma-mode="remap" ! video/x-raw,colorimetry="bt709" conversion you end up with really dark video.

This will require adding the notion of target luminance in the all our video conversion chain (and maybe video-color.h also).