wip : Hud Changes

- get rid of the main static white crosshair permanently.
- render a png overlay to the centre of the screen (make the size large enough that it could display the sort of overlay shown above).
- add an option to the options menu to turn the two moving direction indicators and their attached speed/distance readouts on and off.
- on the main menu, get rid of the ships that appear (so only the background space/nebula shows), and centre the menu buttons and main title