Fix bug where areas under trees tend to appear as voids in the DTM
Fix bug where areas under trees tend to appear as voids in the DTM

Bug arose from original DSM tree filter not working correctly (due to filter
processing mix of input/output data); because it wasn't working properly, only
small holes were created in the DTM (in a checkerboard pattern), which were readily
repaired during the void fill process.  When the tree filter was fixed, larger holes
were created (correctly), but then they could not be void-filled.

The fix is to use the minImage, which has the same voids as the original DSM, but
does not have the tree filter applied, to determine where voids should be in the DTM.
That way, voids where there isn't any data (e.g. over water) remain void, but areas
under trees get filled in.

pubgeo/pubgeo