Add DerefMut for Rect on ll::SDL_Rect (#599)

src/sdl2/rect.rs

@@ -4,7 +4,7 @@
 use sys::rect as ll;
 use std::mem;
 use std::ptr;
-use std::ops::{Deref, Add, BitAnd, BitOr, Div, Mul, Neg, Sub};
+use std::ops::{Deref, DerefMut, Add, BitAnd, BitOr, Div, Mul, Neg, Sub};
 use std::convert::{AsRef, AsMut};
 
 /// The maximal integer value that can be used for rectangles.
@@ -439,6 +439,20 @@ impl Deref for Rect {
     }
 }
 
+impl DerefMut for Rect {
+    /// # Example
+    ///
+    /// ```rust
+    /// use sdl2::rect::Rect;
+    /// let mut rect = Rect::new(2, 3, 4, 5);
+    /// rect.x = 60;
+    /// assert_eq!(60, rect.x);
+    /// ```
+    fn deref_mut(&mut self) -> &mut ll::SDL_Rect {
+        &mut self.raw
+    }
+}
+
 impl Into<(i32, i32, u32, u32)> for Rect {
     fn into(self) -> (i32, i32, u32, u32) {
         (self.raw.x, self.raw.y, self.raw.w as u32, self.raw.h as u32)