input: add a motion signal to weston_pointer

The signal will be emitted after the pointer is moved. A shell plugin
can listen to the signal and activate certain effects when the pointer
touches the screen corners, for instance.

src/compositor.h

@@ -307,6 +307,7 @@ struct weston_pointer {
 	struct weston_view *focus;
 	uint32_t focus_serial;
 	struct wl_signal focus_signal;
+	struct wl_signal motion_signal;
 
 	struct weston_view *sprite;
 	struct wl_listener sprite_destroy_listener;

src/input.c

@@ -392,6 +392,7 @@ weston_pointer_create(void)
 	pointer->default_grab.pointer = pointer;
 	pointer->grab = &pointer->default_grab;
 	wl_signal_init(&pointer->focus_signal);
+	wl_signal_init(&pointer->motion_signal);
 
 	pointer->sprite_destroy_listener.notify = pointer_handle_sprite_destroy;
 
@@ -736,6 +737,8 @@ move_pointer(struct weston_seat *seat, wl_fixed_t x, wl_fixed_t y)
 					 iy - pointer->hotspot_y);
 		weston_view_schedule_repaint(pointer->sprite);
 	}
+
+	wl_signal_emit(&pointer->motion_signal, pointer);
 }
 
 WL_EXPORT void