Make convert_slice_2d() use raw pointers

The util function convert_slice_2d() operates on a slice with stride
information. It will become incompatible with PlaneSlice which will not
expose a multi-rows slice anymore (see
<xiph#631 (comment)>).

To keep it generic enough, we don't want to use a PlaneSlice wrapper for
every call, so make the function use raw pointers (and unsafe).

Note: this commit changes indentation for unsafe blocks, so the diff is
more understable with "git show -b".

src/util.rs

@@ -292,20 +292,19 @@ pub fn round_shift(value: i32, bit: usize) -> i32 {
   (value + (1 << bit >> 1)) >> bit
 }
 
-pub fn convert_slice_2d<NEW, OLD>(
-  dst: &mut [NEW], dst_stride: usize, src: &[OLD], src_stride: usize,
+pub unsafe fn convert_slice_2d<NEW, OLD>(
+  dst: *mut NEW, dst_stride: usize, src: *const OLD, src_stride: usize,
   width: usize, height: usize
 )
 where
   NEW: CastFromPrimitive<OLD> + Copy + 'static,
   OLD: Copy + 'static,
 {
-  for r in 0..height {
-    for (a, b) in dst[r * dst_stride..r * dst_stride + width]
-      .iter_mut()
-      .zip(src[r * src_stride..r * src_stride + width].iter())
-    {
-      *a = NEW::cast_from(*b);
+  for y in 0..height {
+    for x in 0..width {
+      let p_dst = dst.add(y * dst_stride + x);
+      let p_src = src.add(y * src_stride + x);
+      *p_dst = NEW::cast_from(*p_src);
     }
   }
 }