css: differentiate computed value and used value
This makes a difference for some properties, notably for colors (currentColor), for lengths (percentages) and line-height.
The values we store in our GtkCssStyle structs currently are computed values. We kinda have used values for numbers, via the gtk_css_number_value_get() function accepting a 100% value, but we don't treat this systematically, and our implementation of currentColor is defective (see #2620).