Sometimes you want to clear a SculptAttribute handle without
destroying the attribute itself.
Also removed the using namespace blender::bke::paint lines.
* `BKE_sculpt_vertex_attr_get` (and it's alias `SCULPT_vertex_attr_get`)
is now `blender::bke::paint::vertex_attr_ptr`. Same for the face
versions of those functions. The SCULPT_XXX aliases are removed.
* Removed code duplication; `[vertex/face]_attr_ptr` now just calls
a generic template (`elem_attr_ptr`).
* Added `[vertex/face]_attr_get` and `[vertex/face]_attr_set` functions
to get/set attribute data without pointer wrangling.
* Given the unwieldly length of `blender::bke::paint::` it's recommended
to do a `using blender::bke::paint;` at the top of sculpt files.
Example: `float w = vertex_attr_get<float>(vertex,
ss->attrs.automasking_factor);`
Joseph Eagar