lavc/prores: restore pict_type and key_frame setting.
lavfi/blend: always peek the first frame of each queue.

Before the change, filter_frame() always waited for 2 samples in the
bottom frames queue. This notably fixes commands such as
./ffplay tests/lena.pnm -vf split,blend=c0_mode=addition
Changelog: add WebP

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>