Merged support_fills with support_interface_fills.

When extruding supports, the support is interleaved with interface
if possible (when extruded with the same extruder).
Otherwise the base is extruded first.

xs/xsp/ExtrusionPath.xsp

@@ -22,6 +22,7 @@
     void clip_end(double distance);
     void simplify(double tolerance);
     double length();
+    ExtrusionRole role() const;
     bool is_perimeter();
     bool is_infill();
     bool is_solid_infill();
@@ -57,16 +58,6 @@ ExtrusionPath::polyline(...)
     OUTPUT:
         RETVAL
 
-ExtrusionRole
-ExtrusionPath::role(...)
-    CODE:
-        if (items > 1) {
-            THIS->role = (ExtrusionRole)SvUV(ST(1));
-        }
-        RETVAL = THIS->role;
-    OUTPUT:
-        RETVAL
-
 double
 ExtrusionPath::mm3_per_mm(...)
     CODE:
@@ -143,6 +134,7 @@ _constant()
     EXTR_ROLE_SKIRT                        = erSkirt
     EXTR_ROLE_SUPPORTMATERIAL              = erSupportMaterial
     EXTR_ROLE_SUPPORTMATERIAL_INTERFACE    = erSupportMaterialInterface
+    EXTR_ROLE_MIXED                        = erMixed
   PROTOTYPE:
   CODE:
     RETVAL = ix;