canvas: remove unnecessary mut on Canvas::get's self parameter.

rtchallenge/src/canvas.rs

@@ -39,7 +39,7 @@ impl Canvas {
         }
         self.pixels[x + y * self.width] = c;
     }
-    pub fn get(&mut self, x: usize, y: usize) -> Color {
+    pub fn get(&self, x: usize, y: usize) -> Color {
         self.pixels[x + y * self.width]
     }
     pub fn write_to_file<P>(&self, path: P) -> Result<(), CanvasError>