Coalesce animated image frame data into a single shared memory region (#37058)

This makes servo use less file descriptors for animated images and
avoids the crash described in
https://github.com/servo/servo/issues/36792.

Doing this also forces the end users to be more explicit about whether
they want to deal with all image frames or just the first one.
Previously, `Image::bytes` silently returned only the data for the first
frame. With this change there's now a `frames` method which returns an
iterator over all frames in the image.

Testing: No tests - this simply reduces the number of fds used. Servo
doesn't currently display animated gifs anyways.
Fixes: https://github.com/servo/servo/issues/36792

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/compositing/compositor.rs

@@ -1456,10 +1456,11 @@ impl IOCompositor {
                 format: PixelFormat::RGBA8,
                 frames: vec![ImageFrame {
                     delay: None,
-                    bytes: ipc::IpcSharedMemory::from_bytes(&image),
+                    byte_range: 0..image.len(),
                     width: image.width(),
                     height: image.height(),
                 }],
+                bytes: ipc::IpcSharedMemory::from_bytes(&image),
                 id: None,
                 cors_status: CorsStatus::Safe,
             }))