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/webdriver_server/lib.rs

@@ -1709,7 +1709,8 @@ impl Handler {
             "Unexpected screenshot pixel format"
         );
 
-        let rgb = RgbaImage::from_raw(img.width, img.height, img.bytes().to_vec()).unwrap();
+        let rgb =
+            RgbaImage::from_raw(img.width, img.height, img.first_frame().bytes.to_vec()).unwrap();
         let mut png_data = Cursor::new(Vec::new());
         DynamicImage::ImageRgba8(rgb)
             .write_to(&mut png_data, ImageFormat::Png)