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/script/dom/webglrenderingcontext.rs

@@ -619,7 +619,8 @@ impl WebGLRenderingContext {
 
                 let size = Size2D::new(img.width, img.height);
 
-                TexPixels::new(img.bytes(), size, img.format, false)
+                let data = IpcSharedMemory::from_bytes(img.first_frame().bytes);
+                TexPixels::new(data, size, img.format, false)
             },
             // TODO(emilio): Getting canvas data is implemented in CanvasRenderingContext2D,
             // but we need to refactor it moving it to `HTMLCanvasElement` and support