[niepce] rust: fix initialisation of gtk and rexiv2

[Hubert Figuière <hub src gnome org>]

commit e9ae8cc3b7f861440f41e9dfcc14ac172478932d
Author: Hubert Figuière <hub figuiere net>
Date:   Sat Jan 5 16:31:22 2019 -0500

    rust: fix initialisation of gtk and rexiv2

 src/lib.rs | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)
---
diff --git a/src/lib.rs b/src/lib.rs
index 0638867..2f2580b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,7 +1,7 @@
 /*
  * niepce - lib.rs
  *
- * Copyright (C) 2017 Hubert Figuière
+ * Copyright (C) 2017-2019 Hubert Figuière
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -44,8 +44,15 @@ pub mod niepce;
 
 include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
 
-/// Call this to initialize the gtk-rs bindings
+use std::sync::{Once, ONCE_INIT};
+
+/// Call this to initialize rexiv2 and the gtk-rs bindings
 #[no_mangle]
 pub extern "C" fn niepce_init() {
-    gtk::init().unwrap();
+    static START: Once = ONCE_INIT;
+
+    START.call_once(|| unsafe {
+        gtk::init().unwrap();
+        rexiv2::initialize().expect("Unable to initialize rexiv2");
+    });
 }