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.
LINEAR— no easingOUT_QUAD— decelerate (the default for most UI motion)OUT_BACK— decelerate with a slight overshootIN_QUAD— accelerate