The `damaged` path has cleared `Session::held[setup_slot]` since M7, for a reason
that now applies to the failure path too: the slot map says the surface holds THIS
picture while the surface still carries whatever the previous occupant decoded, so
a later `show_existing_frame` naming it blits the old picture's pixels with the old
picture's geometry and colour. The failure path never reached that far before —
`decode_into`'s error returned straight out of `frame_av1` — and the previous commit
made it continue so the slot releases could run.
enricobuehler
5aeb8d2552