Re-worded error messages which were unprecise or unfitting before (model index below 0 was not acknowledged at all, or grouped together with a "too many models" message).
modelIDs are given a default value of -1.
Important: A MODELDEF's FrameIndex lines can no longer refer to model indices that are beyond the number of models of that MODELDEF entry. There is in fact a check to avoid going beyond the number of an actor's models which would abort program operation at startup, but it never caught any such occurances.
surfaceSkinIDs was two-dimensional and is now a one-dimensional TArray as well, elements are accessed via [Row + Column * NumRows], in this case surfaceIndex + modelIndex * MD3_MAX_SURFACES]
Used TArray.Alloc to make TArrays have the correct size depending on the number of models.
Also removed MAX_MODELS_PER_FRAME.
Edited skinSurfaceIDs access for one-dimensional TArray
Added MD3_MODELS_MIN
To ensure compatibility with mods, all model-related TArrays (four in total) have a minimum size of 4, defined by MD3_MODELS_MIN.
