spring: Make clip behavior configurable

When the spring goes outside the envelope, we have a few options for
bringing it back: either just let it overshoot, bounce off the limit or
just clamp it.  Instead of controlling that with #ifdef, let's make it
a part of the spring state.

src/animation.c

@@ -41,6 +41,7 @@ weston_spring_init(struct weston_spring *spring,
 	spring->current = current;
 	spring->previous = current;
 	spring->target = target;
+	spring->clip = WESTON_SPRING_OVERSHOOT;
 }
 
 WL_EXPORT void
@@ -71,22 +72,31 @@ weston_spring_update(struct weston_spring *spring, uint32_t msec)
 			force * step * step;
 		spring->previous = current;
 
-#if 0
+		switch (spring->clip) {
+		case WESTON_SPRING_OVERSHOOT:
+			break;
+
+		case WESTON_SPRING_CLAMP:
 			if (spring->current >= 1.0) {
-#ifdef TWEENER_BOUNCE
-			spring->current = 2.0 - spring->current;
-			spring->previous = 2.0 - spring->previous;
-#else
 				spring->current = 1.0;
 				spring->previous = 1.0;
-#endif
-		}
-
-		if (spring->current <= 0.0) {
+			} else if (spring->current <= 0.0) {
 				spring->current = 0.0;
 				spring->previous = 0.0;
 			}
-#endif
+			break;
+
+		case WESTON_SPRING_BOUNCE:
+			if (spring->current >= 1.0) {
+				spring->current = 2.0 - spring->current;
+				spring->previous = 2.0 - spring->previous;
+			} else if (spring->current <= 0.0) {
+				spring->current = -spring->current;
+				spring->previous = -spring->previous;
+			}
+			break;
+		}
+
 		spring->timestamp += 4;
 	}
 }

src/compositor.h

@@ -113,6 +113,12 @@ struct weston_animation {
 	struct wl_list link;
 };
 
+enum {
+	WESTON_SPRING_OVERSHOOT,
+	WESTON_SPRING_CLAMP,
+	WESTON_SPRING_BOUNCE
+};
+
 struct weston_spring {
 	double k;
 	double friction;
@@ -120,6 +126,7 @@ struct weston_spring {
 	double target;
 	double previous;
 	uint32_t timestamp;
+	uint32_t clip;
 };
 
 enum {