clutter: Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

clutter/clutter/clutter-pan-action.c

@@ -961,6 +961,7 @@ clutter_pan_action_get_motion_delta (ClutterPanAction *self,
       return clutter_pan_action_get_interpolated_delta (self, delta_x, delta_y);
     default:
       g_assert_not_reached ();
+      return 0.0f;
     }
 }
 

clutter/clutter/clutter-shader-effect.c

@@ -333,6 +333,7 @@ clutter_shader_effect_create_shader (ClutterShaderEffect *self)
 
     default:
       g_assert_not_reached ();
+      return COGL_INVALID_HANDLE;
     }
 }
 

clutter/clutter/evdev/clutter-seat-evdev.c

@@ -766,6 +766,7 @@ discrete_to_direction (double discrete_dx,
     return CLUTTER_SCROLL_UP;
   else
     g_assert_not_reached ();
+  return 0;
 }
 
 void