Fixed more cases where attributes weren't being reinitialized
on switching PBVH mode:
* When PBVH_GRIDS and PBVH_BMESH force attributes into simple
array mode they no longer override simple_array in the
SculptAttributeParams parameters, instead they set a field
in SculptAttribute itself. Thus if the attribute is
reinitialized in another mode it won't retain the simple_array
parameter.
* sculpt_attribute_ensure_ex now calls sculpt_attr_update if
the attribute already exists.
* Fixed a bug from a couple commits ago that set
SculptAttribute.data_for_bmesh wrong.
939b63bcd6