When img.src is changed rapidly (e.g., YouTube Music sets a GIF
placeholder then swaps to a real URL via IntersectionObserver), the
failure callback from the stale first request could corrupt the newer
request by calling abort_the_image_request on the now-reassigned
m_current_request.
Fix this by using the existing m_update_the_image_data_count generation
counter to detect stale fetch callbacks.
This fixes thumbnail loading on YouTube Music.
