Added support for better merging of height fields which ignores NoData

values.  Fixed various problems with handing of coordinates systems. Added
support for halving levels in x and y respectively, in addition to the
previous divide in both x and y at the same time, which allows long
line/short tall regions to be handled better.

examples/osgdem/DataSet.h

@@ -669,9 +669,10 @@ class DataSet : public osg::Referenced
             void equalizeCorner(Position position);
             void equalizeEdge(Position position);
             
-
             void equalizeBoundaries();
 
+            void optimizeResolution();
+
             osg::Node* createScene();