Geometry Nodes: Parallelelize a delete geometry selection conversion

Parallelize the conversion of an edge selection to a vertex selection,
which makes it approximately twice as fast (0.6 ms -> 0.3 ms).
In another test the whole node operation became about 17% faster,
from 6.9 to 5.9ms. This only applies to the "All" mode with the
edge selection domain.

Similar to 3d209d1619

Pull Request: https://projects.blender.org/blender/blender/pulls/108541

source/blender/geometry/intern/mesh_copy_selection.cc

@@ -78,7 +78,7 @@ static IndexMask vert_selection_from_edge(const Span<int2> edges,
                                           IndexMaskMemory &memory)
 {
   Array<bool> array(verts_num, false);
-  edge_mask.foreach_index_optimized<int>([&](const int i) {
+  edge_mask.foreach_index_optimized<int>(GrainSize(4096), [&](const int i) {
     array[edges[i][0]] = true;
     array[edges[i][1]] = true;
   });