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.
This should probably be moved into a helper function, exact same code section in `drawLine()`.