Fix#79163 bug related to the bevel operation producing disconnected UVs for
new bevel faces. This change replaces previous approach using scattered and
selective usage of functions: bev_merge_uvs, bev_merge_edge_uvs and
bev_merge_end_uvs with one coherent technique for all stages of the bevel operation.
It is utilizing a concept of loop (BMLoop) buckets to keep track of UV vertices
that should be merged at the end of bevel operation by a single call to
bevel_merge_uvs function. This approach doesn't touch initial UV position
calculation done by interpolation algorithm in bev_create_ngon function and
keeps the concept of representative faces (called frep, facerep or rep_face in
code) to help decide to which bucket specific loops should be assigned.
This is from PR https://projects.blender.org/blender/blender/pulls/139595,
which has more explanation and discussion.
8412c0b42d