Disabled asserts in the release build (-DNDEBUG).

Added a safe variant of offset(const Slic3r::ExPolygon...), which offsets each loop separately.
New functions "remove_sticks" to remove zero area parts of polygons.
New functions "remove_small" and "remove_degenerate" for polygon clean up.
Extended the C++ supports, those are not finalized yet though.

xs/Build.PL

@@ -180,6 +180,9 @@ if ($ENV{SLIC3R_DEBUG}) {
     # only on newer GCCs: -ftemplate-backtrace-limit=0
     push @cflags, '-DSLIC3R_DEBUG';
     push @cflags, $cpp_guess->is_msvc ? '-Gd' : '-g';
+} else {
+    # Disable asserts in the release builds.
+    push @cflags, '-DNDEBUG';
 }
 if ($cpp_guess->is_gcc) {
     # check whether we're dealing with a buggy GCC version