Fix 'implicit fallthrough' warning with new GCC

GCC 7 now warns on case statements falling through without an explicit
comment that falling through is OK. Insert some to make it happy.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>

desktop-shell/shell.c

@@ -345,12 +345,14 @@ get_output_work_area(struct desktop_shell *shell,
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_TOP:
 	default:
 		area->y += panel_height;
+		/* fallthrough */
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_BOTTOM:
 		area->width = output->width;
 		area->height = output->height - panel_height;
 		break;
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_LEFT:
 		area->x += panel_width;
+		/* fallthrough */
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_RIGHT:
 		area->width = output->width - panel_width;
 		area->height = output->height;

tests/subsurface-test.c

@@ -579,22 +579,31 @@ create_subsurface_tree(struct client *client, struct wl_surface **surfs,
 
 	case 11:
 		SUB_LINK(10, 2);
+		/* fallthrough */
 	case 10:
 		SUB_LINK(9, 2);
+		/* fallthrough */
 	case 9:
 		SUB_LINK(8, 6);
+		/* fallthrough */
 	case 8:
 		SUB_LINK(7, 6);
+		/* fallthrough */
 	case 7:
 		SUB_LINK(6, 2);
+		/* fallthrough */
 	case 6:
 		SUB_LINK(5, 1);
+		/* fallthrough */
 	case 5:
 		SUB_LINK(4, 3);
+		/* fallthrough */
 	case 4:
 		SUB_LINK(3, 1);
+		/* fallthrough */
 	case 3:
 		SUB_LINK(2, 0);
+		/* fallthrough */
 	case 2:
 		SUB_LINK(1, 0);