The routine to find dissolvable vertices had a check to ensure
that the vertex was exactly in line with the two neighbors.
I have convinced myself that this check is unneccesary (it was
failing with only a 1e-9 difference from 0 on a cross check),
so have removed it.
Also added code so that exact solver does the whole collection at once.
This patch allows users to use a collection (as an alternative to Object)
for the boolean modifier operand, and therefore get rid of a long modifier stack.
Well, this only fixes the example if one uses the new Exact mode,
but since that is available, seems fair to call this fixed.
Since these were not closed-volume operands, the Exact mode needed
some adjustment to the threshold used for "inside-outside" tests
for the case of deciding if the cutter is inside the other shape
for a Difference.
This is for design task T67744, Boolean Redesign.
It adds a choice of solver to the Boolean modifier and the
Intersect (Boolean) and Intersect (Knife) tools.
The 'Fast' choice is the current Bmesh boolean.
The new 'Exact' choice is a more advanced algorithm that supports
overlapping geometry and uses more robust calculations, but is
slower than the Fast choice.
The default with this commit is set to 'Exact'. We can decide before
the 2.91 release whether or not this is the right choice, but this
choice now will get us more testing and feedback on the new code.
