A bug that was introduced in !131645 where the number of verts eligible
for dissolve was reduced, to prevent dissolving unrelated verts.
That PR changed the code to only do the dissolve check on verts at the
ends of selected edges, which solved bug #109765. However, this didn't
properly account for dissolving only one edge in a chain in a face pair.
This could result in cases where one of the vertices should be checked
but wasn't - if it wasn't selected.
Now when an edge is dissolved, each of its verts is checked,
and if it's in a chain, the VERT_MARK tag is moved down the
chain until it finds its natural endpoint.
Ref !139959.
bfbc9c804c