add AsRef and AsMut for Rect on ll::SDL_Rect

Solves #593

src/sdl2/rect.rs

@@ -5,6 +5,7 @@ use sys::rect as ll;
 use std::mem;
 use std::ptr;
 use std::ops::{Deref, Add, BitAnd, BitOr, Div, Mul, Neg, Sub};
+use std::convert::{AsRef, AsMut};
 
 /// The maximal integer value that can be used for rectangles.
 ///
@@ -450,6 +451,18 @@ impl From<(i32, i32, u32, u32)> for Rect {
     }
 }
 
+impl AsRef<ll::SDL_Rect> for Rect {
+    fn as_ref(&self) -> &ll::SDL_Rect {
+        &self.raw
+    }
+}
+
+impl AsMut<ll::SDL_Rect> for Rect {
+    fn as_mut(&mut self) -> &mut ll::SDL_Rect {
+        &mut self.raw
+    }
+}
+
 // Intersection
 impl BitAnd<Rect> for Rect {
     type Output = Option<Rect>;