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" ... />
MozReview-Commit-ID: IUAK8P07gtm
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extra : rebase_source : d3648101c6f65479b21e6f02945731cd5bb57663
Move from nsStyleColor::CalcComplexColor to StyleComplexColor::CalcColor.
MozReview-Commit-ID: FkYovvPZLc8
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extra : rebase_source : 54f1966e0ef9258f20e954cd6250774008eca04c