Default HDR black point to true zero?

In PQ output mode, we currently default to 0.005 nits out. Even though we send metadata for it (as of !278 (merged) being merged), apparently many environments still don't scale this correctly down to true zero (for e.g. OLED displays). We might need to re-evaluate this default and pick a true black point as default, with the rise of OLED displays.