Add a greyscale mode

[Lorp]

In showing a greyscale environment, this formula should be useful:

For sRGB, once transformed to linear-light, the factors are actually

0.21263900587151027 R + 0.715168678767756 G + 0.07219231536073371 B

Reference: @svgeesus [MPEG-OTSPEC] COLRv1 to gray/alpha question (& color-blindness question)