Add move/scale animation

Add an animation which moves a surface to a new location, at the same
time as also rescaling it to a different size from the origin, rather
than the existing scale animation which resizes from the centre.

[pochu: rebased, ported to weston_view]
diff --git a/src/compositor.h b/src/compositor.h
index ea53f3f..a8504af 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -1298,6 +1298,12 @@
 weston_fade_run(struct weston_view *view,
 		float start, float end, float k,
 		weston_view_animation_done_func_t done, void *data);
+
+struct weston_view_animation *
+weston_move_scale_run(struct weston_view *view, int dx, int dy,
+		      float start, float end, int reverse,
+		      weston_view_animation_done_func_t done, void *data);
+
 void
 weston_fade_update(struct weston_view_animation *fade, float target);