Fixed cracks in MMU painting gizmo. All triangles, even not painted triangles, are now rendered inside MMU painting gizmo.

The cracks were caused by using glPolygonOffset to resolve Z-fighting. All triangles of the object were firstly rendered entirely with a base color. And then paint triangles were drawn over these already rendered triangles.

src/slic3r/GUI/Gizmos/GLGizmoMmuSegmentation.cpp

@@ -99,7 +99,7 @@ void GLGizmoMmuSegmentation::render_painter_gizmo() const
     glsafe(::glEnable(GL_BLEND));
     glsafe(::glEnable(GL_DEPTH_TEST));
 
-    render_triangles(selection);
+    render_triangles(selection, false);
 
     m_c->object_clipper()->render_cut();
     render_cursor();