Sprites, Tilemaps, etc ignore FlxCamera.exists / active

FlxSprite, FlxTilemap and probably the rest of the Flixel objects do not check the state of an FlxCamera before drawing to its buffer. It should probably check either "exists" or "active" (or both?). As it stands if you create 3 cameras but only want one of them visible and actually rendering, the Flixel objects will still render to all 3 regardless if the cameras are active or not.
