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.
diff --git a/src/compositor.h b/src/compositor.h
index a56bd4d..12ec8e3 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -113,6 +113,12 @@
 	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 @@
 	double target;
 	double previous;
 	uint32_t timestamp;
+	uint32_t clip;
 };
 
 enum {