Fix for to clipper::union without enough offset

[supermerill]

with prusaslicer 2.3.1:
If I try to slice this object with a modifier:
clipper_error_test.zip
then some extremely tiny holes are created and ruin the print
I've created this pullrequest to repair this error
It allow clipper::union to not add extremely small holes if called with safety_offset
It check that all polygons are all thicker than scaled_epsilon
It may be a bit costly.. i don't know by how much vs the union. It may use more int and less double with some careful refactoring...

[bubnikv]

The issue is caused by slice simplification.

// Simplify slices if required.
if (m_print->config().resolution)
    this->simplify_slices(scale_(this->print()->config().resolution));

[bubnikv]

Fixed by refactoring the slicing algorithm vs. xy offset compensation order.
Thanks for pointing out the issue.