The code was trying to ignore hidden geometry when doing boolean,
which is correct when used as a tool, but not when a modifier.
Added a "keep_hidden" argument to bmesh_boolean to distinguish the
two cases.
Also fixed a bug when the tool is used with hidden geometry that
is attached to unhidden geometry that is deleted by the operation.
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.
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.
