GRIM/EMI: Cleanup

- add override to overridden functions
- make sure that all overridden functions are virtual
- make local functions protected
EMI: Clear depth buffer before starting to draw the Overworld

- all actors of the Overworld should cover any non-Overworld drawings
- Overworld actors need to use the depth buffer so that e.g. the
pause screen is drawn above the inventory

- this fixes issue #927 (inventory is drawn above background of pause
  or menu screen)
EMI: Change frustum parameters for Overworld actors

- lower the distance to the near plane of the frustum for Overworld
  and change the other parameters accordingly

- this makes the following actors visible:
 - background of directions to Pegnose Pete's house (issue #869)
 - Murray in the murray_go easter egg
 - the trophies during the diving contest (upper left corner)
 - the screenshots for the save games (still with the wrong texture)