Cleanup static variables from FlxPoint / FlxRect / FlxMatrix

[gamedevsam]

These variables should be stored where they are used: https://github.com/HaxeFlixel/flixel/blob/dev/flixel/math/FlxPoint.hx#L16-L19
https://github.com/HaxeFlixel/flixel/blob/dev/flixel/math/FlxRect.hx#L13-L15

We should think about cleaning up this type of API bloat for this major release.

[Gama11]

Make them private or remove them, using pooling instead?

[gamedevsam]

I would use pooling, or in cases where the points are hit every frame, store them as static variables on the class that actually uses them.

[Gama11]

I've already done the cleanup, see the referenced commits. It's just the static flash objects that are leftover (but made private).