Added touch-overlay filtering ability

	* You can now specify a touch_overlay:filter_rect in the engine.xml settings
	* This filter rect is a positive-match filter, meaning any touches inside this rectangle are considered valid.
	* Anything outside this rectangle will be discarded by the TouchManager
	* If your app overrides any of the onTouchsBegin/onTouchesMoved/onTouchesEnd methods, then you should check:

		mEngine.shouldDiscardTouch( translatedTouchPosition)
		... after calling mEngine.translateTouchPoint(...)