Various Sprite/Texture fixes

Use independent depth buffer for Overworld drawings:
- fix issue #927 (inventory is drawn above background of pause or menu screen)

Fix Frustum parameters for Overworld actors:
- fix issue #869 (background of directions to Pegnose Pete's house missing)
Missing override.

Also, the rest of the class is missing override. Not sure how I missed that when I added override to things. Maybe don't add override and I'll fix it for the whole file later.