SGModel animation changes

This deletes a load of unused animation code, and adds fixes from @jaj22 for #1970 and to allow multiple levels of detail to have animations.

Note: fix for #1970 needs testing by someone who can reproduce the original bug!

The reason for allowing animation on lower levels of detail is that the difference between gear-up and gear-down may be visually quite obvious even when the ship is fairly small on screen, but animations tend to involve many separately rendered components (which creates a lot of overhead). So if a landing gear animation is necessary even at lower LOD, it's better to use a simplified version.