Adding object elevation param.

Quick attempt to build with MinGW 7.3. Successful compile, failed linking

src/libslic3r/SLA/SLASupportTree.hpp

@@ -33,7 +33,7 @@ struct SupportConfig {
     double head_front_radius_mm = 0.2;
 
     // How much the pinhead has to penetrate the model surface
-    double head_penetraiton_mm = 0.5;
+    double head_penetration_mm = 0.5;
 
     // Radius of the back side of the 3d arrow.
     double head_back_radius_mm = 0.5;