hud scripting interface changes

- functions that draw or set hud state are written as hudaction
- functions that get hud state are written as hud:thing
- coordinate system is now:
  - center screen is 0,0
  - top center is 0,-1000
  - bottom center is 0,1000
  - x coordinates are the same size as y coordinates,
    so -1000,-1000 to 1000,1000 is a square
  - the aspect ratio width/height can be accessed with the hud:aspect function
  - hud:left and hud:right are provided as convenience functions for
    getting the x coordinates of the left and right edges of the screen