Follow ground rotation on flat ground

[ihm-tswow]

@Varen

Found when investigating #102 and likely what's causing it. Will push a patch for that, but not sure if there's something else that should be fixed here.

Reproduce (works on 3354 and 3085, won't be noticable after that):

1. Open a map with flat ground (blades edge arena)
2. In the object editor, make sure "rotate following cursor" is selected
3. Drag any object on completely flat ground
4. Get this love letter in the log:

21813 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21816 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21819 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21821 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21824 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21826 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21829 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21831 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21833 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21836 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21838 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21841 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21843 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21846 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21848 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21851 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21853 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21855 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21858 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21860 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21862 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21864 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21867 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21869 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed
21876 - (World.cpp:652): [Error] rotate_selected_models_to_ground_normal ray intersection failed

[ihm-tswow]

temporary fix: 2729bd8