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.