Optimization of G-code export:

1) Don't allocate ExtruderOverrides if not necessary
2) Use boost::container::small_vector<int32, 3) for ExtruderOverrides
   (usually less than 4 instances are printed))

src/libslic3r/pchheader.hpp

@@ -63,6 +63,7 @@
 #include <boost/bind.hpp>
 #include <boost/config.hpp>
 #include <boost/config/warning_disable.hpp>
+#include <boost/container/small_vector.hpp>
 #include <boost/date_time/local_time/local_time.hpp>
 #include <boost/date_time/posix_time/posix_time.hpp>
 #include <boost/filesystem.hpp>