Improved accuracy of slicing (triangle cutting) code,

improved debugging outputs and asserts of the slicing code.
Disabled detection of concave corners with horizontal faces,
as too often there were found models with badly triangulated faces,
see for example GH issue #895.

xs/src/libslic3r/Utils.hpp

@@ -9,6 +9,7 @@ namespace Slic3r {
 
 extern void set_logging_level(unsigned int level);
 extern void trace(unsigned int level, const char *message);
+extern void disable_multi_threading();
 
 // Set a path with GUI resource files.
 void set_var_dir(const std::string &path);