FlxSpriteUtil curveTo + fixes, etc

Added `curveTo()`.  The default filling behavior caused the curves to always be filled with a line drawn from end to start to complete the figure (`graphics.beginFill + graphics.curveTo` default).  Changed filling to create a default `fillStyle` if null in the drawShapes, which maintains the previous behavior of a null `fillStyle`.  A null `fillStyle` now means `beginFill` doesn't get called--happens in `drawLine` and optionally in `drawCurve`.

**Breaking:** removed the `Color` param of `beginDraw`.  Either `fillStyle.color` was overriding it, or it didn't get used.  I doubt anyone used this function (should be private imo), so it's not a big deal.  Added `fillStyle.alpha` to the fill which probably was forgotten in the FlxColor rework.

Consistency in `drawCircle` params.

I don't like `Color` and `FillStyle` both as params, and some params start with a caps letter while others don't (class names), but those are issues for another time.
Seems like fillStyle is kinda useless, if Color can be both the color and alpha fields. Only useful thing is hasFill. Will fix in a bit.