color: relax reference_lum requirements

Prompted by !78 (comment 2561733) .

The case where reference white luminance would be higher than the primary color volume maximum luminance is the Windows-style linear scRGB. (1.0, 1.0, 1.0) is 80 cd/m², but the reference white could e.g. match the PQ system's 203 cd/m² ~ (2.5, 2.5, 2.5).