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.cpp

@@ -24,6 +24,8 @@
 #include <boost/nowide/integration/filesystem.hpp>
 #include <boost/nowide/convert.hpp>
 
+#include <tbb/task_scheduler_init.h>
+
 namespace Slic3r {
 
 static boost::log::trivial::severity_level logSeverity = boost::log::trivial::error;
@@ -82,6 +84,14 @@ void trace(unsigned int level, const char *message)
         (::boost::log::keywords::severity = severity)) << message;
 }
 
+void disable_multi_threading()
+{
+    // Disable parallelization so the Shiny profiler works
+    static tbb::task_scheduler_init *tbb_init = nullptr;
+    if (tbb_init == nullptr)
+        tbb_init = new tbb::task_scheduler_init(1);
+}
+
 static std::string g_var_dir;
 
 void set_var_dir(const std::string &dir)