Flip it if you can't collapse it

[sloriot]

if a needle is also a cap, it will be handled as a needle only. But if we can't collapse the edge, a flip will not be tried. Try the flip too.

Note that this will slow down the method as the status of a triangle is tested twice.

Fix #8605

[MaelRL]

Do you not fill both ranges on purpose? There's still:

  ...
  else // let's not make it possible to have a face be both a cap and a needle (for now)
  ...

a little below; so, if it's a needle & a cap, you fill the needle range, but not the cap range.

[MaelRL]

I'm not sure about the current changes in the is_badly_shaped + collect functions.

I think it should be something like:

collect almost degenerate triangles():
- if it's a needle
-   put it in the needle range
- else if it's a cap
-   put it in the cap range

treat():
- while there are needles or caps:
-   if there are needles, pop the topmost needle
-     if it's no longer a needle or collapse is rejected
-      test if it's a cap // this is what was missing
-        put it in caps
-   if there are caps
-    pop the cap, treat it 

So:

• no edge should appear in both ranges
• there shouldn't be any calls to the version of is_badly_shaped() that checks for both needle & cap regardless of needle-ness.

I think you didn't need to modify as much, it should be enough with the lines filling the edges_to_flip range and leave is_badly_shaped() + collection untouched?