FIX: avoid arrange to nonprefered region

Previously we assume the NFP of exclude regions are rectangle, which was
not always right. Now we calculate the NFP and find a best new position
to shift.

Change-Id: I02c075603cf71dd3c9146d7ac7a6706c0f850669
(cherry picked from commit 713ebd666c90d5dcfaf89914c37d211e9a470e99)

src/libslic3r/ExPolygon.cpp

@@ -183,6 +183,25 @@ bool overlaps(const ExPolygons& expolys1, const ExPolygons& expolys2)
         }
     }
     return false;
+}
+
+Point projection_onto(const ExPolygons& polygons, const Point& from)
+{
+    Point projected_pt;
+    double min_dist = std::numeric_limits<double>::max();
+
+    for (const auto& poly : polygons) {
+        for (int i = 0; i < poly.num_contours(); i++) {
+            Point p = from.projection_onto(poly.contour_or_hole(i));
+            double dist = (from - p).cast<double>().squaredNorm();
+            if (dist < min_dist) {
+                projected_pt = p;
+                min_dist = dist;
+            }
+        }
+    }
+
+    return projected_pt;
 }
 
 void ExPolygon::simplify_p(double tolerance, Polygons* polygons) const