Blender Git Loki

Fix grid clipping resulting in empty trees.

Issue was that the grids didn't have a transform set when clipping was
happening. As the clip operator copies the transform (here an invalid
one) from the grid to the mask if their tranforms mismatch, the
operation resulted in an invalid/empty tree.

(Best explanation I can give so far.)

Also for some reason exporting the obstacles field doesn't work anymore,
so it is disabled for now, and so is clipping for it as it doesn't make
sense to clip the obstacles based on the density field.