Revert "Making self mutable"

This reverts commit 15a2fd7d1a.

src/wand/magick.rs

@@ -241,7 +241,7 @@ impl MagickWand {
 
     /// Rotate the currently selected image by the given number of degrees,
     /// filling any empty space with the background color of a given PixelWand
-    pub fn rotate_image(&mut self, background: &PixelWand, degrees: f64) -> Result<(), &'static str> {
+    pub fn rotate_image(&self, background: &PixelWand, degrees: f64) -> Result<(), &'static str> {
         match unsafe { bindings::MagickRotateImage(self.wand, background.wand, degrees) } {
             bindings::MagickBooleanType::MagickTrue => Ok(()),
             _ => Err("failed to rotate image")