This was caused when the BKE_pbvh_draw_cb function was used with
update_only_visible set to false. In that case, all nodes with the flag
were updating, but the update flag was only cleared for visible nodes.
This was causing constant updates per redraw in no visible nodes until
they enter the view frustum and their flag was cleared.

In order to fix this and prevent it from happening again:

• Updating the buffers, flushing the updates and clearing the flags are

now part of the same function. It does not make sense to do these in
separate places.

• The BKE_pbvh_draw_cb function was refactored so the

pbvh_update_draw_buffers is only called once. It should now be easier to
understand what the function does when it is used to update only visible
nodes or all nodes.