Wipe tower priming lines are placed at origin with custom bed shapes

Custom shapes were previously detected as circular and the lines were placed off the bed

src/libslic3r/GCode/WipeTower.hpp

@@ -277,7 +277,8 @@ private:
     // Bed properties
     enum {
         RectangularBed,
-        CircularBed
+        CircularBed,
+        CustomBed
     } m_bed_shape;
     float m_bed_width; // width of the bed bounding box
     Vec2f m_bed_bottom_left; // bottom-left corner coordinates (for rectangular beds)