colorspace: update interpretation of SDR white

Based on ITU-R Report BT.2408, and general recommendations within the
industry, the "SDR white level", i.e. the level at which to map SDR into
HDR signals, is not 100 cd/m^2 but a value closer to 203 cd/m^2.

For PQ signals this results in a relatively straightforward change of
the code, but for HLG signals things get more complicated. For HLG,
rather than targeting a fixed brightness in cd/m^2, the recommendation
is to map SDR white levels to the 75% point in HLG, which calculates to
a value of about roughly 3.17955 in scene-referred space (where the
nominal peak is at 12.0). To fit this into the libplacebo interpretation
of these values (where 1.0 maps to the SDR white levels), we scale
things down by this factor, giving rise to a new scene-referred signal
range of 12.0/3.17955 = 3.774, and adjust the OOTF to compensate.

This commit does put into question to what extent the default tone
mapping settings need to be altered to account for this change in
interpretation when going from HDR back to SDR.

Also add tests to ensure this stuff round-trips.

Bump the API version because, apart from the fact that we changed a
public header, this is quite a drastic change in functionality.