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.
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