Absolute vs relative luminance

[palemieux]

float values outside of [0,1] range can be used to represent colors outside of the chosen color gamut.

Care will be needed here since at least one popular HDR color systems, i.e. ITU Rec 2100 PQ, maps to absolute linear light values, which range from 0 to 10,000.

In this specific case, it will be necessary to specify that the canvas value are for normalized linear colour values to use the term from ITU Rec 2100 PQ.

[palemieux]

See #23