Add AnimatedDecodedImageData which implements DecodedImageData with
an 8-slot buffer pool instead of storing all frames in memory.
Frames are requested on demand from the ImageDecoder service as
the animation progresses.
For a 344-frame animated image at 1920x1080, this reduces
WebContent memory from ~1.3 GB to ~66 MB.
The streaming class owns frame progression and synchronizes
multiple callers (HTMLImageElement and ImageStyleValue) through
notify_frame_advanced() returning the authoritative frame index.
When a frame isn't in the pool, the last displayed frame is shown
as a fallback (brief freeze rather than blank).
Rename the old AnimatedBitmapDecodedImageData (which now only
handles static/single-frame images) to BitmapDecodedImageData.
