canvas: Use Cow<[u8]> for bytes() getter (#37249)

I fell into trap of over-generalization in
https://github.com/servo/servo/pull/36793, but
https://github.com/servo/servo/pull/36821 showed `Cow<[u8]>` is all we
need (to reuse existing vec alloc or pass on a slice).

Testing: There are WPT tests, but it's just refactor so rust keeps us
safe.
Split of https://github.com/servo/servo/pull/36821

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/canvas/canvas_data.rs

@@ -1327,7 +1327,7 @@ impl<'a, B: Backend> CanvasData<'a, B> {
                     .to_vec()
             }
         } else {
-            self.drawtarget.bytes().as_ref().to_vec()
+            self.drawtarget.bytes().into_owned()
         };
 
         Snapshot::from_vec(