Avoid use of Plane::p in Plane::downsample

Plane::p accesses pixels in a fashion that the compiler
cannot assume to be linear. This is harmful to both
compiler optimization and cache linearity.

Although we cannot completely eliminate this problem
since two rows must be accessed at a time for downsampling,
we can alleviate it by accessing the data slice directly.

Improves performance of av-scenechange by 15%.
Did not benchmark rav1e but presumably helps by 1-2% there also.

v_frame/src/plane.rs

@@ -424,7 +424,6 @@ impl<T: Pixel> Plane<T> {
     }
   }
 
-  #[allow(clippy::needless_range_loop)]
   pub fn downsampled(
     &self, frame_width: usize, frame_height: usize,
   ) -> Plane<T> {
@@ -443,23 +442,26 @@ impl<T: Pixel> Plane<T> {
 
     let width = new.cfg.width;
     let height = new.cfg.height;
-    let xorigin = new.cfg.xorigin;
-    let yorigin = new.cfg.yorigin;
-    let stride = new.cfg.stride;
 
     assert!(width * 2 <= src.cfg.stride - src.cfg.xorigin);
     assert!(height * 2 <= src.cfg.alloc_height - src.cfg.yorigin);
 
-    for row in 0..height {
-      let base = (yorigin + row) * stride + xorigin;
-      let dst = &mut new.data[base..base + width];
-
-      for (col, dst) in dst.iter_mut().enumerate() {
+    let data_origin = src.data_origin();
+    for (row_idx, dst_row) in new
+      .mut_slice(PlaneOffset::default())
+      .rows_iter_mut()
+      .enumerate()
+      .take(height)
+    {
+      let src_top_row = &data_origin[(src.cfg.stride * row_idx * 2)..];
+      let src_bottom_row =
+        &data_origin[(src.cfg.stride * (row_idx * 2 + 1))..];
+      for (col, dst) in dst_row.iter_mut().take(width).enumerate() {
         let mut sum = 0;
-        sum += u32::cast_from(src.p(2 * col, 2 * row));
-        sum += u32::cast_from(src.p(2 * col + 1, 2 * row));
-        sum += u32::cast_from(src.p(2 * col, 2 * row + 1));
-        sum += u32::cast_from(src.p(2 * col + 1, 2 * row + 1));
+        sum += u32::cast_from(src_top_row[2 * col]);
+        sum += u32::cast_from(src_top_row[2 * col + 1]);
+        sum += u32::cast_from(src_bottom_row[2 * col]);
+        sum += u32::cast_from(src_bottom_row[2 * col + 1]);
         let avg = (sum + 2) >> 2;
         *dst = T::cast_from(avg);
       }