Split the external infill pattern to separate top / bottom config values.

Based on f9344a00e3
thanks @supermerill
Implements  #475, #479, #1133, #1474

src/libslic3r/Surface.cpp

@@ -42,6 +42,12 @@ Surface::is_internal() const
         || this->surface_type == stInternalVoid;
 }
 
+bool
+Surface::is_top() const
+{
+    return this->surface_type == stTop;
+}
+
 bool
 Surface::is_bottom() const
 {