Fixed|Resources: Crash when loading/rendering models (boxed jDRP)

There were two issues after the removal of the fixed submodel limit:

1) A model definition may have no submodels at all, but the code still
accesses submodel #0 in some places, causing a crash.

2) The stateModefs array contained pointers, but as the modeldefs are
now stored in a dynamic array, existing pointers are invalidated when
the array grows. The stateModefs array now stores indexes instead of
pointers.