Add preview of the Camera2D's screen boundaries

Created by: bojidar-bg

Closes #3365 (closed)

This implementation does not apply smoothing nor drag margins to the preview, which might be misleading in some cases (e.g. you still have to make sure the player can see what is displayed). This is not much of an issue though, since most of the time you would either position the camera at a starting location, or you would animate a non-interpolating camera around.