Fixed update optimisations

Now that I’ve had to reduce the physics update time-step, any code in my fixed update functions is much more performance critical.  So this week I tried optimising as much of it as I could.  Unfortunately there weren’t really any places I could move update code out of the fixed update altogether, but I did manage to speed up a few things.

I’ll probably have to revisit fixed update performance at some point, but for now I’m gonna move onto the next thing.  Next up I think I’ll look at reworking the system that determines how parts can attach together, to make it more flexible and allow for more interesting part pairing relationships.  This’ll be required for some of the new parts I want to add in the future.