* gradient, colours: use 24-bit colour depth internally on Wayland
our Colour type assumes this anyway; lower colour depth should be handled by display backends
* core: restore gradient color computation
this was mistakenly disabled when moving to a unified Colour representation
* core, gradient: use Colour type
* tests: gradient: always test hsv/hcl
colour computations no longer depend on the display backends we compile with, so these can be tested unconditionally now
* gradients, specials: use Color type
this makes specials now accept color names as well as hex codes, and removes all notions of color depth from gradient calculations
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Co-authored-by: bi4k8 <bi4k8@github>