Deprecated.since 1.4 (Carbide 2.0), this variant is unsafe, since the model
is no longer locked during a save, so the document map from a model
might be changing.
set the mask depth (usually 1,2,4,8 or 0 for none)
NOTE: for SVG, if this is nonzero, it doesn't mean a mask file is present,
merely that an enum will be generated.