Refactored StyleComplexColor to support "complex" blending between
background (numeric) color and foreground color (currentColor).
Made explicit the distinction between numeric, currentColor and a
complex blend in Gecko and Stylo.

This is to support SMIL animation, for example, of the form:

     <animate from="rgb(10,20,30)" by="currentColor" ... />

Bug: 1465307
Reviewed-by: hiro,xidorn
MozReview-Commit-ID: IUAK8P07gtm