Correctly handle reduced Text size

Fixed another problem on `Text`, which is when a `Text` is given a shorter `text`, the `width` does not reflect the new size because `_source` is not changed:

```haxe
import com.haxepunk.*;
import com.haxepunk.graphics.*;

class TestWorld extends World {
	override public function begin():Void {
		super.begin();
		
		var text = new Text("TestingTestingTestingTesting", { resizable: true });
		trace(text.textWidth + " " + text.textHeight + " " + text.width + " " + text.height); //196 16 196 16
		text.text = "Testing";
		trace(text.textWidth + " " + text.textHeight + " " + text.width + " " + text.height); //52 16 196 16 :(
		
		addGraphic(Image.createRect(text.textWidth, text.textHeight, 0xFF0000));
		addGraphic(text);
	}
}

class Main extends Engine {
	public function new():Void {
		super();
		HXP.world = new TestWorld();
	}
	
	static public function main():Void {
		nme.Lib.current.addChild(new Main());
	}
}
```

I think it is time to create a proper unit test. It is because the recent improvements of HaxePunk broke quite a few things. I am still having another seemly unrelated problem on Android, which I am going to investigate...