- Only do this if PBVH is not used for display, this should correspond
to whats' happening in sculpt.c now.
- No need to do such synchronization for solid drawing, it supports
optimal display from PBVH.
In fact, such synchronization is only needed in the following case:
Drawing callback does not support PBVH draw and sculpt session is
configured to use PBVH for display and related operations.