BMesh: improve handling of custom-data flag (Mesh.cd_flag)

Code that handled merging & initializing custom-data from other
meshes sometimes missed checks for this flag, causing bevel weights to
lost when the mesh was converted to a BMesh.

The following changes are a more general fix for T94197.

- Add BM_mesh_copy_init_customdata_from_mesh_array which initializes
  custom-data from multiple meshes at once.
  As well as initializing custom-data layers from Mesh.cd_flag.

  This isn't essential for boolean, however it avoids the overhead of
  resizing custom-data layers.

- Loading mesh data into a BMesh now respects Mesh.cd_flag
  instead of only checking if the BMesh custom-data-layer exists.
  Without this, the order of meshes passed to BM_mesh_bm_from_me could
  give different (incorrect) results.

- Copying mesh data now copies `cd_flag` too. This is a precaution
  as in my tests evaluating modifiers these values always matched.
  Nevertheless it's correct to copy this value as custom-data it's
  self is being copied.

1 files changed, 2 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 1aa3477437a..9b0cf17a424 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -150,6 +150,8 @@ static void mesh_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
 
   BKE_mesh_update_customdata_pointers(mesh_dst, do_tessface);
 
+  mesh_dst->cd_flag = mesh_src->cd_flag;
+
   mesh_dst->edit_mesh = nullptr;
 
   mesh_dst->mselect = (MSelect *)MEM_dupallocN(mesh_dst->mselect);