Better Vector docstrings and some bugfixes

- __sub__ actually works
- norm() uses mag() instead of len() to allow it to work with decimal
magnitudes.
Method to convert pitch/yaw/roll to unit vector

Useful when determining direction of an instance or similar things.
Partial implementation of glass/grating instance

- This will change the single glass_128x128 instance to a different one
for gratings, so the ents can be different.