Descent 1 mangles colors during `g3_init_polygon_model`, so this must
not be called on polygons not designed for mangling. Rearrange the
logic to allow Descent 1 to verify that polygon models are well-formed
without using the functions that mangle the colors.
Fixes: 42a2e3ab0b ("Avoid crash loading polymodels with invalid subcalls")
Reported-by: derhass <https://github.com/dxx-rebirth/dxx-rebirth/issues/416>