WIP: Refactored bridging flow from normal flow, new config value

'thick_bridges' to switch between the Slic3r vs. S3D/Cura/Ideamaker
way of printing 1st object layer over supports.
Simplified the PresetHints.

src/libslic3r/PrintObject.cpp

@@ -554,6 +554,7 @@ bool PrintObject::invalidate_state_by_config_options(
             || opt_key == "perimeter_extrusion_width"
             || opt_key == "infill_overlap"
             || opt_key == "thin_walls"
+            || opt_key == "thick_bridges"
             || opt_key == "external_perimeters_first") {
             steps.emplace_back(posPerimeters);
         } else if (
@@ -1459,14 +1460,7 @@ void PrintObject::bridge_over_infill()
         if (region.config().fill_density.value == 100) continue;
         
         // get bridge flow
-        Flow bridge_flow = region.flow(
-            frSolidInfill,
-            -1,     // layer height, not relevant for bridge flow
-            true,   // bridge
-            false,  // first layer
-            -1,     // custom width, not relevant for bridge flow
-            *this
-        );
+        Flow bridge_flow = region.bridging_flow(frSolidInfill);
         
 		for (LayerPtrs::iterator layer_it = m_layers.begin(); layer_it != m_layers.end(); ++ layer_it) {
             // skip first layer
@@ -1488,7 +1482,7 @@ void PrintObject::bridge_over_infill()
                 Polygons to_bridge_pp = internal_solid;
                 
                 // iterate through lower layers spanned by bridge_flow
-                double bottom_z = layer->print_z - bridge_flow.height;
+                double bottom_z = layer->print_z - bridge_flow.height();
                 for (int i = int(layer_it - m_layers.begin()) - 1; i >= 0; --i) {
                     const Layer* lower_layer = m_layers[i];