[librsvg] marker.rs: Add a type definition for a MarkerRenderFn

[Federico Mena Quintero <federico src gnome org>]

commit 717a9d141d8abee9847c2e381e36b21f297a4058
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Mar 2 13:02:56 2017 -0600

    marker.rs: Add a type definition for a MarkerRenderFn
    
    This will take a marker type (start/middle/end) and all the parameters
    for rendering a marker.

 rust/src/marker.rs |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)
---
diff --git a/rust/src/marker.rs b/rust/src/marker.rs
index a9d8208..6792195 100644
--- a/rust/src/marker.rs
+++ b/rust/src/marker.rs
@@ -573,6 +573,19 @@ fn bisect_angles (incoming: f64, outgoing: f64) -> f64 {
     }
 }
 
+// From SVG's marker-start, marker-mid, marker-end properties
+enum MarkerType {
+    Start,
+    Middle,
+    End
+}
+
+type RenderMarkerFn = FnOnce (MarkerType,
+                              f64,  // xpos
+                              f64,  // ypos
+                              f64,  // computed_angle
+                              f64); // line_width
+
 fn emit_marker_by_name (draw_ctx:       *const RsvgDrawingCtx,
                         marker_name:    *const libc::c_char,
                         xpos:           f64,