Fixed painting gizmos with modifiers below the bed:

- When a modifier was below the bed and all the object parts above, it would clip
  the modifier but not triangulate the cut.
- When an object part was below, it would triangulate all modifiers with
  opaque orange color.

Both should now be fixed.

src/slic3r/GUI/3DScene.hpp

@@ -39,6 +39,10 @@ class ModelObject;
 class ModelVolume;
 enum ModelInstanceEPrintVolumeState : unsigned char;
 
+// Return appropriate color based on the ModelVolume.
+std::array<float, 4> color_from_model_volume(const ModelVolume& model_volume);
+
+
 // A container for interleaved arrays of 3D vertices and normals,
 // possibly indexed by triangles and / or quads.
 class GLIndexedVertexArray {
@@ -393,6 +397,7 @@ public:
         return out;
     }
 
+    void set_color(const std::array<float, 4>& rgba);
     void set_render_color(float r, float g, float b, float a);
     void set_render_color(const std::array<float, 4>& rgba);
     // Sets render color in dependence of current state