* clutter/clutter-alpha.c:
	(clutter_alpha_get_alpha): Do not return the cached value but
	just compute the current alpha value. The use case is pretty
	much straightforward: stop a timeline, advance it, and retrieve
	the alpha value before starting the timeline to adjust an
	animation dependent on the alpha. Caching the alpha value is
	not needed because the behaviours will always pass the alpha
	value inside the ::alpha-notify virtual function anyway, so
	there is no need for them to call clutter_alpha_get_alpha().

	This change makes it also possible to reliably unit-test
	ClutterAlpha.

	(timeline_new_frame_cb): Call clutter_alpha_get_alpha().