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/backend.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use std::borrow::Cow;
+
 use canvas_traits::canvas::{
     CompositionOrBlending, FillOrStrokeStyle, LineCapStyle, LineJoinStyle,
 };
@@ -21,7 +23,6 @@ pub(crate) trait Backend: Clone + Sized {
     type DrawTarget: GenericDrawTarget<Self>;
     type PathBuilder: GenericPathBuilder<Self>;
     type SourceSurface;
-    type Bytes<'a>: AsRef<[u8]>;
     type Path: PathHelpers<Self> + Clone;
     type GradientStop;
     type GradientStops;
@@ -122,7 +123,7 @@ pub(crate) trait GenericDrawTarget<B: Backend> {
         draw_options: &B::DrawOptions,
     );
     fn surface(&self) -> B::SourceSurface;
-    fn bytes(&'_ self) -> B::Bytes<'_>;
+    fn bytes(&self) -> Cow<[u8]>;
 }
 
 /// A generic PathBuilder that abstracts the interface for azure's and raqote's PathBuilder.

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(

components/canvas/raqote_backend.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use std::borrow::Cow;
 use std::cell::RefCell;
 use std::collections::HashMap;
 
@@ -41,7 +42,6 @@ impl Backend for RaqoteBackend {
     type DrawTarget = raqote::DrawTarget;
     type PathBuilder = PathBuilder;
     type SourceSurface = Vec<u8>; // TODO: See if we can avoid the alloc (probably?)
-    type Bytes<'a> = &'a [u8];
     type Path = raqote::Path;
     type GradientStop = raqote::GradientStop;
     type GradientStops = Vec<raqote::GradientStop>;
@@ -656,9 +656,11 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
         );
     }
     #[allow(unsafe_code)]
-    fn bytes(&self) -> &[u8] {
+    fn bytes(&self) -> Cow<[u8]> {
         let v = self.get_data();
-        unsafe { std::slice::from_raw_parts(v.as_ptr() as *const u8, std::mem::size_of_val(v)) }
+        Cow::Borrowed(unsafe {
+            std::slice::from_raw_parts(v.as_ptr() as *const u8, std::mem::size_of_val(v))
+        })
     }
 }