[libdazzle] animation: cleanup ease in/out cubic

[Christian Hergert <chergert src gnome org>]

commit 6adad4c4c5c245cfc8e8add1f7656317f059bb60
Author: Christian Hergert <chergert redhat com>
Date:   Fri Aug 4 10:04:35 2017 -0700

    animation: cleanup ease in/out cubic

 src/animation/dzl-animation.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/src/animation/dzl-animation.c b/src/animation/dzl-animation.c
index 7e74bf0..aa0d1c5 100644
--- a/src/animation/dzl-animation.c
+++ b/src/animation/dzl-animation.c
@@ -143,10 +143,12 @@ dzl_animation_alpha_ease_out_cubic (gdouble offset)
 static gdouble
 dzl_animation_alpha_ease_in_out_cubic (gdouble offset)
 {
-  if (offset < .5)
-    return dzl_animation_alpha_ease_in_cubic (offset * 2.0) / 2.0;
-  else
-    return .5 + dzl_animation_alpha_ease_out_cubic ((offset - .5) * 2.0) / 2.0;
+  gdouble p = offset * 2.0;
+
+  if (p < 1.0)
+    return 0.5 * p * p * p;
+  p -= 2.0;
+  return 0.5 * (p * p * p + 2.0);
 }