Fix #112351: Sculpt implicit sharing thread safety crash

Currently the iteration over a PBVH node's vertices retrieves mutable
access to the mask custom data layer. This isn't threadsafe, but it is
done in the multithreaded loops over all nodes.

In general, we need to be more careful and conservative about storage
of non-const pointers to mesh data. Ideally we would only have one
mutable reference to a resource at a time. And we should avoid doing
work like looking up custom data layers more than we need to.

To that end, make the pointer to the custom data layer used everywhere
const, and retrieve mutable access before parallel node iteration with
a specific function, and write to the mask data with that in mind.

This pushes us in the direction of sharing less code per PBVH type.
In my opinion that's a good thing, because we can actually optimize for
each type. For example, `SCULPT_mask_write_array` gives a picture of
how each of these hot loops could become much simpler.

@@ -502,3 +504,3 @@
   int totvert;
   const int *vert_indices;
-  float *vmask;
+  const float *vmask;

@@ -2718,3 +2718,3 @@
   for (int i = 0; i < data->totlayer; i++) {
     if (data->layers[i].type == type) {
-      BLI_assert(uint(n) < uint(layer_num));
+      BLI_assert(n < layer_num);

@@ -74,3 +74,3 @@
     {0}};
 
-static void mask_flood_fill_set_elem(float *elem, PaintMaskFloodMode mode, float value)
+static float mask_flood_fill_set_elem(const float elem, PaintMaskFloodMode mode, float value)

@@ -122,1 +121,3 @@
-        if (prevmask != *vi.mask) {
+        const float new_mask = mask_flood_fill_set_elem(prevmask, mode, value);
+        if (prevmask != new_mask) {
+          SCULPT_mask_vert_set(BKE_pbvh_type(ob->sculpt->pbvh), mask_write, new_mask, vi);

@@ -1205,3 +1200,1 @@
-    BKE_pbvh_vertex_iter_end;
-    BKE_pbvh_node_mark_redraw(node);
-  }
+  SCULPT_mask_write_array(ss, nodes, {expand_cache->original_mask, SCULPT_vertex_count_get(ss)});