Remove alpha from contrast calc

[Synchro]

#1724 highlighted a misunderstanding of how the contrast function works. I'd thought of this at the time I wrote the contrast function, and I'd incorporated the alpha value into the calculation. This is evidently wrong, because we can't know the background colour, so we have no idea whether the alpha will lighten or darken the colour it's comparing. So the alpha term should be removed from the calculation altogether. While this will affect current users, current output is actively wrong so I don't think it's a BC break, just a bug fix.

[seven-phases-max]

+1
It probably can even throw a warning when transparent color is used as contrast argument(s) (if we'll have a "warning facility")

[lukeapage]

we have the start of a warning facility...

[seven-phases-max]

@lukeapage do you mean console.warn like here? Yes, I guess this is the start, though it still needs some goodies to show line/column it is rised from (or do I miss something?).

[Synchro]

Good idea - separate ticket though? OK to merge this?

[lukeapage]

@seven-phases-max yes.. it really is a tentative start!