SGModel animation changes

Changes:

* A load of unused animation code has been deleted.
* Adds a fix from @jaj22 for #1970
* Multiple meshes can now have animations covering the same time range, and these will be mapped to the same named animation in the game. This means that things like landing gear animations can be included in multiple levels of detail, and should work seamlessly through detail level changes. Modeller note: You should just have one `anim` definition line for each named animation, they don't need to be duplicated for each mesh or detail level. This means that each version/part of the animation should have the same start & end points.
* `Animation::Interpolate` no longer does looping; that can be done at the driving level (looping the animation time value) instead if necessary.
* Internally, all keyframe times are now converted to seconds.

Note: fix for #1970 should be tested 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.