Blender Git Loki

transform_snap_object: Better bvhtree creation management for editing multiple objects.

- Use the object referenced in `BMEditMesh` as the `ghash` key to save the bvhtrees in cache;
- Create a boundbox around edit_mesh to test the snap before creating bvhtree;
- Save the `edit_mesh`s bvhtree in the mesh bvh_cache;

This is a part of the D3504.