Some Clipper optimizations
Use linestring_length() from Boost
Align rectilinear and line infill across layers. #712
Revert "Align rectilinear and line infill across layers. #712"

This reverts commit a0a54ea70697418ae6fb7939490dcf3c29ecd845.
Update unit test after recent conversion of Boost::Geometry::Utils to double coordinates
Align rectilinear and line infill across layers (new implementation). #712
Minor optimization
Merge branch 'master' into clipper-optimizations
Fix syntax for ex_int_offset2()
Merge branch 'master' into boost-optimizations
Fix initialization of Rectilinear filler
Cache Slic3r::Fill object
Move bridge detection to its own method
TYpo
Fix dependencies to require Math::Clipper 1.18.
To avoid error "Undefined subroutine &Math::Clipper::int_offset ...".
Use binary search for get_layer_range.
Gives a 13% speed up on Teethy_Tiki.stl (thing:49334) with default settings
on my laptop.
When $self->layers->[0]->slice_z == $min_z the returned $min_layer value is
0 rather than the (incorrect?) -1 returned by the old code.
Avoid merging meshes when there is only one mesh to merge.
When slicing yoda.stl it reduces peak memory usage by 5% and speeds up
slicing by 13%.
Further optimization using a simple proxy to implement the two methods
(size/extends) needed on the merge meshes is also possible and
undoubtedly worthwhile.
Avoid memory leaks by breaking reference cycle: Print -> Fill -> Print
Bugfix: cooling buffer was broken. #1149