Model loader: improve instancing

Previously: create one Graphics::Surface per assimp aiMesh, which are then added into Graphics::StaticMeshes during Loader::ConvertNodes phase. The idea being, that if an assimp node uses multiple aiMeshes they are merged under one StaticMesh (and multiple SMs if the total exceeds 65k indices).
But, in reality multiple aiMeshes in one assimp node pretty much did not happen, and duplicate meshes would still get their own vertex buffers.

Now: just create one StaticMesh+StaticGeometry node per aiMesh. Simpler, saves memory.

No changes required to existing models, of course.