fix(video-cover): double the ANMF frame offset (node-webpmux quirk)
node-webpmux returns the raw WebP frame X/Y, which the spec stores as actual/2 (frame offsets are
always even); libwebp/webpmux double it. We placed partial frames at HALF offset -> the moving
sub-frame ghosted over the base frame (a doubled/garbled torus on Mastodon). x2 the x/y offset so
frames land at their true pixel position. Verified against the libwebp anim_dump reference: the
composited frame now matches.
Co-Authored-By: Claude <noreply@…>