Revert "Optimization: simplify fill_surfaces before the offset operation"

To workaround issue #1325 and possibly #1320.

This reverts commit 3a046e34111aeaf3701062f006e9f5957635e15b.
One more test about polygon simplification
Reapply correct optimization for simplifiying fill_surfaces before performing the offset. #1325
Implement lower_bound() and upper_bound() methods for ZTable
Missing #include causing XS build to fail. #1349
Fix garbage collection of shared data. #1348
Update MANIFEST*
Temporary workarond for an upstream bug in Moo which causes failure when running with threads. #1330
Smarter ordering of gap fill
Bugfix: medial axis thin wall detection was triggered when not needed
Remove t/freeze.t. #1352
Bugfix: only_retract_when_crossing_perimeters was not triggering retraction when moving between islands that are covered/bridged on the above layer. #1308
Don't merge adjacent bridges so that more correct angles can be detected for each one
Scale layer height ranges too when scaling object in plater. #1284
Fix little regression causing object height not to be updated when object in plater was scaled
Better name for wipe
Can't NULL floats...
Perform a ./Build distclean in ./xs automatically
Extend utils/dump-stl.pl to also write STL files
Failing test case about spurious infill in hollow objects, caused by shells being correctly generated even for hollow objects - however sometimes we don't want that