Bug identified in #110942.
In cases where the last corners of a polygon are out of context, the
`weld_iter_loop_of_poly_next` iterator skips these corners.
This means that some corners of the resulting mesh do not have the
value set, which can even result in infinite loops.
The solution was not simple. The iterator had to be practically redone
to not use the `loop_end` member (which caused the problem).
Fortunately the code is more simplified with this change.
262b67d61d