Resolves the CSS custom property by walking element's ancestor
chain (self included) and returns the first SvgPaintServer the value
maps to. Falls back to FallbackServer (or None)
when the property is not defined anywhere in the hierarchy.
public SvgPaintServer Resolve(SvgElement element)
Circular variable references (e.g. --a: var(--a)) are not detected and will
cause a System.StackOverflowException. Such declarations are invalid per the
CSS specification and the variable is treated as unset by conforming browsers.