Added parameter "support_buildplate_only" for SLA supports.

src/libslic3r/SLA/SLASupportTree.hpp

@@ -55,6 +55,9 @@ struct SupportConfig {
     // How to connect pillars
     PillarConnectionMode pillar_connection_mode = PillarConnectionMode::dynamic;
 
+    // Only generate pillars that can be routed to ground
+    bool ground_facing_only = false;
+
     // TODO: unimplemented at the moment. This coefficient will have an impact
     // when bridges and pillars are merged. The resulting pillar should be a bit
     // thicker than the ones merging into it. How much thicker? I don't know