Exposed by 6c774feba2
`BM_mesh_decimate_dissolve_ex` sets up `DelimitData` with layer offset
(start), size and end so that `bm_edge_is_contiguous_loop_cd_all` can
check a range of edges for being contiguous.
The way `cd_loop_offset_end` is calculated is wrong though, it does not
take the actual start into account (this has to be added to fix the
bug). When it is wrong, it can happen that start and end are the same,
so no check actually takes place and no delimiting edges are found.
It seems that prior to 6c774feba2 the customdata layer always had an
offset of zero, so never really showed in practice (at least I couldnt
make it break in 3.4), but after 6c774feba2 we can at least observe the
following:
- when creating a bmesh, an offset would to the uv layer would still be
zero in my tests
- however, as soon as we iterate loops of a face (as done in the
report), we get an additional layer `CD_BM_ELEM_PYPTR`
- this then changes the offset
- `BM_uv_map_get_offsets_from_layer` seems to do the right thing afaict
So to resolve, just add the "start" offset to the end, to get the right
range.
NOTE: there is a very similar `DelimitData` used in
`bmesh.ops.join_triangles` and the way in which `bm_edge_delimit_cdata`
sets up te range is exactly like what this PR proposes.
Philipp Oeser