Update object bounding box during sculpt

Fixes the sculpt object being incorrectly clipped during drawing due
to an out-of-date bounding box making it seem that the object had gone
outside the view.

Added a BKE_pbvh function to get the top-level bounding box. In
sculpt_flush_update(), where the PBVH bounds are updated, the result
is copied to the object's bounding box.

Fixes bug [#33790]
projects.blender.org/tracker/?func=detail&aid=33790&group_id=9&atid=498

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53914 954f8c5b-7b00-dc11-b283-0030488c597c