added preliminary support for MERGE_ALG=ADD for heatmaps

git-svn-id: https://svn.osgeo.org/gdal/trunk@26182 f0d54148-0727-0410-94bb-9a71ac55c965
added preliminary support for MERGE_ALG=ADD for heatmaps

git-svn-id: https://svn.osgeo.org/gdal/trunk@26183 f0d54148-0727-0410-94bb-9a71ac55c965