Moved the call to HeightFieldUtils::resolveInvalidHeights()

from ElevationLayer::populateHeightField() to ElevationPool::getOrCreateRaster().
This resolves an issue where if you used a CompositeElevationLayer
along with a GDALDEM layer you would get areas colored in that should be
transparent because CompositeElevationLayer was calling populateHeightField
under the hood which was resolving NODATA_VALUE to 0 before rasterizing.

src/osgEarth/ElevationLayer.cpp

@@ -1242,9 +1242,6 @@ ElevationLayerVector::populateHeightField(
     }
 #endif
 
-    // Resolve any invalid heights in the output heightfield.
-    HeightFieldUtils::resolveInvalidHeights(hf, key.getExtent(), NO_DATA_VALUE, 0L);
-
     if (progress && progress->isCanceled())
     {
         return false;

src/osgEarth/ElevationPool.cpp

@@ -360,6 +360,9 @@ ElevationPool::getOrCreateRaster(
                 map->getElevationInterpolation(),
                 progress);
 
+            // Resolve any invalid heights in the output heightfield.
+            HeightFieldUtils::resolveInvalidHeights(hf.get(), keyToUse.getExtent(), NO_DATA_VALUE, 0L);
+
             if ((populated == true) ||
                 (acceptLowerRes == false) ||
                 (progress && progress->isCanceled()))