Fix 3D object ordering when opacity is reduced

Reducing opacity by any amount causes front 3D objects in Mayavi to appear behind deeper surfaces from some angles. Turn on depth peeling after rendering 3D surfaces or points to fix this ordering as described in this issue: enthought/mayavi#491 .

magmap/gui/vis_3d.py

@@ -158,7 +158,9 @@ def plot_3d_points(self, roi, channel, flipz=False):
                 pts.module_manager.scalar_lut_manager.lut.table = cmap(
                     range(0, 256)) * 255
             self._resize_glyphs_isotropic(settings, pts)
-
+
+        # keep visual ordering of surfaces when opacity is reduced
+        self.scene.renderer.use_depth_peeling = True
         print("time for 3D points display: {}".format(time() - time_start))
         return True
 
@@ -278,7 +280,9 @@ def plot_3d_surface(self, roi, channel, segment=False, flipz=False):
 
             self._resize_glyphs_isotropic(settings, surface)
             surfaces.append(surface)
-
+
+        # keep visual ordering of surfaces when opacity is reduced
+        self.scene.renderer.use_depth_peeling = True
         print("time to render 3D surface: {}".format(time() - time_start))
         self.surfaces = surfaces
         return surfaces