PLCombine() combines two polylists into a single polylist and
returns the new polylist. If either of the old polylists was NULL,
a copy of the other polylist (or NULL if both polylists were NULL)
is returned. PLCombine() will generate an error if either of the
parameters was not of the polylist type.
If the two polylists represent color in an inconsistant way, PLCombine
will attempt to preserve the color data. If either polylist has
per-polygon colors, the result will have per-polygon colors. If the
other object originally had per-vertex colors, the color of the first
point of each polygon will be used as the color of the polygon. If the
other polylist originally had no colors, the rgba value 0.15, 0.15, 0.15, 1.0
will be used as the color of each of that polylines polygons.
If neither of the polylists had per-polygon colors but one had per-vertex
colors, the value 0.15, 0.15, 0.15, 1.0 will be used as the color
value for each of the vertices of the colorless polylist.
If neither polylist had color data, the result will not have color
The result of PLCombine() will not include normal data if the two
polylists represent normals in an inconsistent way, for example,
if one has per-vertex data and one has per-face data or one has
per-vertex data and one has no data.