Add is_within_visible_xy implementation

[tjdcs]

Add an xy implementation to is_within_visible_spectrum for brightness invariant checking.

[KelSolaar]

I think that I would prefer a dedicated separate public definition instead of magic based on the shape. What about something like renaming the current one to is_within_visible_spectrum_volume and the xy one is_within_visible_spectrum_area?

Cheers,