timeline: Add cubic-bezier() progress functions

Another progress function from the CSS3 Transitions specification, using
a parametrices cubic bezier curve between (0, 0) and (1, 1) with two
control points.

(sadly, no ASCII art can approximate a cubic bezier, so no graph)

The cubic-bezier() progress function comes with a bunch of preset easing
modes: ease, ease-in, ease-out, and ease-in-out, that we can map to
enumeration values.

clutter/clutter-easing.h

@@ -129,6 +129,13 @@ G_GNUC_INTERNAL
 double  clutter_ease_steps_end          (double t,
                                          double d,
                                          int    steps);
+G_GNUC_INTERNAL
+double  clutter_ease_cubic_bezier       (double t,
+                                         double d,
+                                         double x_1,
+                                         double y_1,
+                                         double x_2,
+                                         double y_2);
 
 G_END_DECLS