Fix #145762: Ensure a valid background value before pruning SDF grid

OpenVDB will throw an exception when trying to prune a SDF/Level-Set grid with a
negative background value.

The geometry nodes in this case are constructing a nominal SDF grid from a
generic "density" volume cube. The negative background value results from
processing the background value the same way as all other grid values in the new
field+grid math feature (`blender::nodes::process_background`).

Since any grid can be constructed with a negative background value we have to
check and correct this before calling the OpenVDB prune function.

Pull Request: https://projects.blender.org/blender/blender/pulls/146837

source/blender/geometry/intern/volume_grid_resample.cc

@@ -28,6 +28,11 @@ openvdb::FloatGrid &resample_sdf_grid_if_necessary(bke::VolumeGrid<float> &volum
 
   /* TODO: Using #doResampleToMatch when the transform is affine and non-scaled may be faster. */
   openvdb::tools::resampleToMatch<openvdb::tools::BoxSampler>(grid, *storage);
+  /* Ensure valid background value for level set grids, otherwise pruning will throw an exception.
+   */
+  if (storage->background() < 0.0f) {
+    storage->tree().root().setBackground(0.0f, true);
+  }
   openvdb::tools::pruneLevelSet(storage->tree());
 
   return *storage;