The easing curve for a UITween.
An enum abstract over Int, so it is just an Int at runtime (no allocation) while giving
named, type-safe values. Where a UIEase is expected the name can be written unqualified, e.g.
UITween.to(setP, 0, 1, 155, OUT_QUAD); elsewhere use UIEase.OUT_QUAD. from/to Int keep
it interchangeable with plain ints.
The four originals (LINEAR/OUT_QUAD/OUT_BACK/IN_QUAD) keep their historical ids; the
rest are the standard Penner families in IN_* (accelerate), OUT_* (decelerate) and
IN_OUT_* (both) variants. OUT_QUAD is the sensible default for most UI motion.