This is not optimal as it will cause minor graphical glitches for
Irrlicht 1.7 users, but at least it fixes the build.

This should fix #1844. Thanks to oleastre for making the first version
of this commit (#1848).

- Don't create and cache an extruded mesh for every (non-node) item.
  Instead use a single one per image resolution.

- For cubic nodes reuse a single wield mesh too

- Improve lighting of the wielded item

- Increase far value of wield mesh scene camera, fixes #1770

- Also includes some minor refactorings of Camera and GenericCAO.