The inverse transfer equation was generated by colorspace.js.
Code that calculates inverse equation is broken with variable
multiplication, so have manually calculated the inverse.
Luminance needed upscaling - not sure whether it's a HDR thing or
float/int encoding thing.
Forward does 128.0 => 1.4376, inverse 1.4376 => 127.99
From tests brightness looks/feels fine when multiplied by 192.