Apply bug fix from upstream clipper
Clipper fix in commit [r463] for bug repport #92 (http://sourceforge.net/p/polyclipping/code/463/tree//trunk/cpp/clipper.cpp?diff=504b9404fd48f873331e913b:462)
is applied here.
Merge pull request #1953 from ledvinap/fix-clipper-1834

Apply bug fix from upstream clipper. Fixes #1834
Merge branch 'master' of git://github.com/alexrj/slic3r