Fix spurious black pixels occuring during buffer accumulation (due to invalid color values)

viewer/glow-extras/viewer/renderables/MeshRenderable.cc

@@ -159,6 +159,7 @@ void MeshRenderable::init()
                                         fColor.rgb = vColor.rgb;
                                     }
                                     fColor.a = 1;
+                                    fColor = clamp(fColor, 0., 1.);
                                  )");