protected static void setGeometryStartIndex(Geometry geom, int startIndex) {
        if (startIndex < -1) {
            throw new AssertionError();
        }
        geom.startIndex = startIndex;
    }
An abstract class for implementations that perform grouping of geometries
via instancing or batching.

@author Kirill Vainer
jme3-core/src/main/java/com/jme3/scene/GeometryGroupNode.java