Enable letterboxing by adjusting HXP.screen.x and HXP.screen.y

This commit allows you to move HXP.screen.x and HXP.screen.y in order to re-position the game screen. HXP.screen and HXP.engine both need to be moved at the same time so that cursor positions and drawing positions are adjusted correctly. I use this for letterboxing to adjust the screen size as the window is resized while maintaining the aspect ratio.

This also moves the debug console's mask outlines into the correct position. I haven't tested this yet, but I suspect this will fix https://github.com/HaxePunk/HaxePunk/issues/235.

![screenshot - 04112014 - 10 49 06 pm](https://cloud.githubusercontent.com/assets/544977/2686181/9330d694-c1ed-11e3-8c8b-938ac90b3946.png)