[librsvg/rustification] drawing_ctx.rs: Put the declaration of RsvgDrawingCtx here

[Federico Mena Quintero <federico src gnome org>]

commit 4eca2e07b6419d32e8d45ff71cf60aa24b908217
Author: Federico Mena Quintero <federico gnome org>
Date:   Wed Nov 16 15:40:13 2016 -0600

    drawing_ctx.rs: Put the declaration of RsvgDrawingCtx here
    
    We'll use this file to declare the accessors to an RsvgDrawingCtx, which
    are right now implemented in C.

 rust/src/drawing_ctx.rs |   16 ++++++++++++++++
 rust/src/lib.rs         |    5 +++--
 rust/src/marker.rs      |    3 +--
 3 files changed, 20 insertions(+), 4 deletions(-)
---
diff --git a/rust/src/drawing_ctx.rs b/rust/src/drawing_ctx.rs
new file mode 100644
index 0000000..2a16d82
--- /dev/null
+++ b/rust/src/drawing_ctx.rs
@@ -0,0 +1,16 @@
+pub enum RsvgDrawingCtx {}
+
+extern "C" {
+    fn rsvg_drawing_ctx_get_view_box_size (draw_ctx: *const RsvgDrawingCtx,
+                                           out_x: *mut f64,
+                                           out_y: *mut f64);
+}
+
+pub fn get_view_box_size (draw_ctx: *const RsvgDrawingCtx) -> (f64, f64) {
+    let mut w: f64 = 0.0;
+    let mut h: f64 = 0.0;
+
+    unsafe { rsvg_drawing_ctx_get_view_box_size (draw_ctx, &mut w, &mut h); }
+
+    (w, h)
+}
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index 200f5d6..731d230 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -26,8 +26,9 @@ pub use length::{
     rsvg_length_hand_normalize,
 };
 
+mod drawing_ctx;
+mod length;
+mod marker;
 mod path_builder;
 mod path_parser;
-mod marker;
 mod strtod;
-mod length;
diff --git a/rust/src/marker.rs b/rust/src/marker.rs
index 71b497c..c1d7de2 100644
--- a/rust/src/marker.rs
+++ b/rust/src/marker.rs
@@ -3,6 +3,7 @@ extern crate cairo;
 extern crate cairo_sys;
 
 use path_builder::*;
+use drawing_ctx::RsvgDrawingCtx;
 
 #[derive(Debug, PartialEq)]
 pub enum Segment {
@@ -291,8 +292,6 @@ fn angle_from_vector (vx: f64, vy: f64) -> f64 {
     }
 }
 
-pub enum RsvgDrawingCtx {}
-
 extern "C" {
     fn rsvg_marker_render (marker_name: *const libc::c_char,
                            xpos: f64,