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/xsp/XS.xsp

@@ -48,6 +48,11 @@ trace(level, message)
     CODE:
         Slic3r::trace(level, message);
 
+void
+disable_multi_threading()
+    CODE:
+        Slic3r::disable_multi_threading();
+
 void
 set_var_dir(dir)
     char  *dir;