[compositing-1] Lighter compositing operator as a blend mode

[bugnano]

On the https://drafts.fxtf.org/compositing-1/ document, the mix-blend-mode and background-blend-mode CSS properties support only blend modes and not Porter Duff compositing operations, and that's perfectly OK.

What amazes me is that there is a lack of a simple additive operation as far as blend modes are concerned. Sure, there is the 'lighter' Porter Duff compositing operation that does exactly that. Too bad that, being a composite operation and not a blend mode, it is not possible to use that operation in CSS mix-blend-mode and background-blend-mode.

My suggestion would be to either treat the 'lighter' function as a blend mode, or add the equivalent operation to the blend modes, with a different name.

One could argue that in doing so, there is a redundancy regarding the add operation, but as far as redundancies are concerned, so is for example the opacity filter in regard to the opacity CSS property, that both do the exact same thing, but in different contexts.

[jakearchibald]

I think this can be closed now that plus-lighter has landed

[jakearchibald]

#444