spring: Make min/max part of spring parameters

Don't hard code the 0.0 - 1.0 spring envelope.
diff --git a/src/compositor.h b/src/compositor.h
index 12ec8e3..faa2f20 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -125,6 +125,7 @@
 	double current;
 	double target;
 	double previous;
+	double min, max;
 	uint32_t timestamp;
 	uint32_t clip;
 };