replace common index code

src/buffer.rs

@@ -16,10 +16,7 @@ impl Buf {
 
     // Truncate x and y and return a corresponding index into the data slice.
     const fn index(&self, x: f32, y: f32) -> usize {
-        // x/y can come in negative, hence we shift them by width/height.
-        let i = (x + self.width as f32) as usize & (self.width - 1);
-        let j = (y + self.height as f32) as usize & (self.height - 1);
-        j * self.width + i
+        crate::util::index(self.width, self.height, x, y)
     }
 
     // Get the buffer value at a given position. The implementation effectively treats data as periodic, hence any finite position will produce a value.