[librsvg/rustification] marker.rs: Normalize negative angles into positive ones

[Federico Mena Quintero <federico src gnome org>]

commit 605408a2abeda045fb26a553f9ca2bf546f76eec
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon Dec 19 16:25:02 2016 -0600

    marker.rs: Normalize negative angles into positive ones
    
    This way our averaging of angles will work.

 rust/src/marker.rs |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/rust/src/marker.rs b/rust/src/marker.rs
index 2426cbf..10f2170 100644
--- a/rust/src/marker.rs
+++ b/rust/src/marker.rs
@@ -2,6 +2,7 @@ extern crate libc;
 extern crate cairo;
 extern crate cairo_sys;
 
+use std::f64;
 use path_builder::*;
 use drawing_ctx::RsvgDrawingCtx;
 
@@ -302,11 +303,15 @@ fn find_outgoing_directionality_forwards (segments: &[Segment], start_index: usi
 }
 
 fn angle_from_vector (vx: f64, vy: f64) -> f64 {
-    let angle = vy.atan2 (vx);
+    let mut angle = vy.atan2 (vx);
 
     if angle.is_nan () {
         0.0
     } else {
+        while angle < 0.0 {
+            angle += f64::consts::PI * 2.0;
+        }
+
         angle
     }
 }