harden tinycolor.equals

In rare cases a stale version of a `tinycolor` gets passed to the `.equals` method, which causes a TypeError (`TypeError: Object #<Object> has no method 'toRgbString'`).
This is because even though a passed in color (e.g. stale JSON variant) qualifies for `typeof color == "object" && color.hasOwnProperty("_tc_id")` although it does not have the according `.toRgbString`.