model paging patch from Till Busch

model paging patch from Till Busch

From Till:
i started the project at the end of february with a simple idea: move all
3d-model loading to the DatabasePager-thread. my first attempts looked
promising, though they were a little too optimistic (or naive?). the patch
has evolved a lot since.

currently it does the following things:
1. revive SGModelLib, move functions for xml-model-loading there

2. replace all calls to sgLoad3dModel with calls to either
SGModelLib::loadModel() or SGModelLib::loadPagedModel()
almost all models will be loaded by the DatabasePager. the few exceptions are:
your own plane, shared models in scenery, random objects, AIBallistic models.

3. simplify mode-loading functions (avoid passing around fg_root)

4. avoid supurious MatrixTransform nodes in loaded models

5. fix some memory leaks