FlxSprite.isSimpleRender has huge side effects and causes a batch flushes

FlxSprite.isSimpleRender has huge side effects and causes a batch flush on EVERY call.

Make the function simple and readable, and defer render state
handling to where we already have it, in FlxSprite.draw().

Tested on my own games that uses rotating sprites and 1 shader.
Blending and other fancyness not tested, but I think it will work.

Fixes #78