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

src/backends/meta-monitor-manager.c

@@ -994,6 +994,7 @@ get_connector_type_name (MetaConnectorType connector_type)
     case META_CONNECTOR_TYPE_DSI: return "DSI";
     default: g_assert_not_reached ();
     }
+  return NULL;
 }
 
 static GList *

src/backends/meta-monitor-transform.c

@@ -37,4 +37,5 @@ meta_monitor_transform_invert (MetaMonitorTransform transform)
       return transform;
     }
   g_assert_not_reached ();
+  return 0;
 }

src/backends/meta-remote-desktop-session.c

@@ -479,6 +479,7 @@ discrete_steps_to_scroll_direction (unsigned int axis,
     return CLUTTER_SCROLL_RIGHT;
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static gboolean

src/backends/native/meta-cursor-renderer-native.c

@@ -924,6 +924,7 @@ is_cursor_hw_state_valid (MetaCursorSprite *cursor_sprite,
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 #ifdef HAVE_WAYLAND

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -173,6 +173,7 @@ meta_monitor_transform_to_xrandr (MetaMonitorTransform transform)
     }
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static gboolean

src/backends/x11/meta-renderer-x11.c

@@ -70,6 +70,7 @@ get_x11_cogl_winsys_vtable (CoglRenderer *renderer)
       break;
     }
   g_assert_not_reached ();
+  return NULL;
 }
 
 static CoglRenderer *

src/compositor/meta-background.c

@@ -635,6 +635,7 @@ texture_has_alpha (CoglTexture *texture)
       return FALSE;
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }
 

src/compositor/meta-surface-actor.c

@@ -348,6 +348,7 @@ meta_surface_actor_is_argb32 (MetaSurfaceActor *self)
       return FALSE;
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }
 

src/core/boxes.c

@@ -1246,6 +1246,7 @@ meta_rectangle_edge_aligns (const MetaRectangle *rect, const MetaEdge *edge)
              BOX_LEFT (edge->rect) <= BOX_RIGHT (*rect);
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }
 

src/core/display.c

@@ -1603,6 +1603,7 @@ get_event_route_from_grab_op (MetaGrabOp op)
 
     default:
       g_assert_not_reached ();
+      return 0;
     }
 }
 
@@ -2622,6 +2623,7 @@ meta_display_supports_extended_barriers (MetaDisplay *display)
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 /**

src/core/edge-resistance.c

@@ -308,6 +308,7 @@ movement_towards_edge (MetaSide side, int increment)
       return increment > 0;
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }
 

src/core/events.c

@@ -86,6 +86,7 @@ get_window_for_event (MetaDisplay        *display,
       return display->grab_window;
     default:
       g_assert_not_reached ();
+      return NULL;
     }
 }
 

src/core/util.c

@@ -992,6 +992,7 @@ meta_get_locale_direction (void)
       return META_LOCALE_DIRECTION_RTL;
     default:
       g_assert_not_reached ();
+      return 0;
     }
 }
 

src/core/window.c

@@ -5498,6 +5498,7 @@ meta_window_get_workspaces (MetaWindow *window)
     return NULL;
   else
     g_assert_not_reached ();
+  return NULL;
 }
 
 static void

src/ui/frames.c

@@ -955,6 +955,7 @@ grab_op_from_resize_control (MetaFrameControl control)
       return META_GRAB_OP_RESIZING_W;
     default:
       g_assert_not_reached ();
+      return META_GRAB_OP_NONE;
     }
 }
 
@@ -1003,6 +1004,7 @@ get_button_number (const ClutterEvent *event)
     return clutter_event_get_button (event);
 
   g_assert_not_reached ();
+  return -1;
 }
 
 static gboolean
@@ -1105,6 +1107,7 @@ meta_frame_left_click_event (MetaUIFrame        *frame,
       return FALSE;
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }
 

src/wayland/meta-wayland-buffer.c

@@ -373,6 +373,7 @@ meta_wayland_buffer_attach (MetaWaylandBuffer *buffer,
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 CoglTexture *

src/wayland/meta-wayland-outputs.c

@@ -94,6 +94,7 @@ cogl_subpixel_order_to_wl_output_subpixel (CoglSubpixelOrder subpixel_order)
     }
 
   g_assert_not_reached ();
+  return WL_OUTPUT_SUBPIXEL_UNKNOWN;
 }
 
 static enum wl_output_subpixel

src/wayland/meta-wayland-xdg-shell.c

@@ -1832,6 +1832,7 @@ positioner_anchor_to_placement_anchor (uint32_t anchor)
       return (META_PLACEMENT_ANCHOR_BOTTOM | META_PLACEMENT_ANCHOR_RIGHT);
     default:
       g_assert_not_reached ();
+      return META_PLACEMENT_ANCHOR_NONE;
     }
 }
 
@@ -1860,6 +1861,7 @@ positioner_gravity_to_placement_gravity (uint32_t gravity)
       return (META_PLACEMENT_GRAVITY_BOTTOM | META_PLACEMENT_GRAVITY_RIGHT);
     default:
       g_assert_not_reached ();
+      return META_PLACEMENT_GRAVITY_NONE;
     }
 }
 

src/x11/iconcache.c

@@ -302,6 +302,7 @@ standard_pict_format_for_depth (int depth)
     default:
       g_assert_not_reached ();
     }
+  return 0;
 }
 
 static XRenderPictFormat *

src/x11/window-x11.c

@@ -931,6 +931,7 @@ is_edge_constraint_resizable (MetaEdgeConstraint constraint)
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 static gboolean
@@ -946,6 +947,7 @@ is_edge_constraint_tiled (MetaEdgeConstraint constraint)
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 static unsigned long