wip: css: Fix computation of currentcolor

We must always store NULL for currentcolor in our values structs, since that is resolved to the color of the style at use time, it can't be fixed at compute time, since values structs are shared between styles.

The last commit in the branch changes things to implement "currentColor computes to currentColor".

To make this work fully, we need probably need to get rid of color expressions first, since currentColor occurring deep inside such expressions messes up the late evaluation we use here.

Another problem for the approach are all the places where we get the rgba value out of a css value without a full style around, e.g. in the shadow code.