Update to Tree Iterator to work with Mask Grids

Signed-off-by: Dan Bailey <danbailey@ilm.com>

Author: danrbailey

openvdb/openvdb/tree/Iterator.h

@@ -83,7 +83,7 @@ class IteratorBase
     /// @details If this iterator is pointing to a child node, then the current item
     /// in the parent node's table is required to be inactive.  In that case,
     /// this method has no effect.
-    void setValueOff() const { parent().mValueMask.setOff(this->pos()); }
+    void setValueOff() const { parent().setValueMask(this->pos(), false); }
 
     /// Return the coordinates of the item to which this iterator is pointing.
     Coord getCoord() const { return parent().offsetToGlobalCoord(this->pos()); }