Add separate arrangement distance control for SLA mode

src/slic3r/GUI/GLCanvas3D.hpp

@@ -389,6 +389,7 @@ public:
     {
         float distance           = 6.;
         float distance_seq_print = 6.;    // Used when sequential print is ON
+        float distance_sla       = 6.;
         float accuracy           = 0.65f; // Unused currently
         bool  enable_rotation    = false;
     };