Hollowing: randomize hole mesh translations before unification.

To prevent self intersections in the output mesh.

src/libslic3r/SLAPrintSteps.hpp

@@ -1,6 +1,8 @@
 #ifndef SLAPRINTSTEPS_HPP
 #define SLAPRINTSTEPS_HPP
 
+#include <random>
+
 #include <libslic3r/SLAPrint.hpp>
 
 #include <libslic3r/SLA/Hollowing.hpp>
@@ -12,6 +14,7 @@ class SLAPrint::Steps
 {
 private:
     SLAPrint *m_print = nullptr;
+    std::mt19937 m_rng;
     
 public:    
     // where the per object operations start and end