When dissolving an edge merges faces, use an angle threshold before
dissolving vertices from the face which have become chains as reult
of the merge (connected to 2 edges).
Also fix edge-flag handling when dissolving multiple edges
from a chain into a single edge, previously flags from the
resulting edge was effectively random.
Now flags from all edges are merged.
Resolves#100184.
Ref !134017
e418f7b1f1