dav1d: drain instead of failing on EAGAIN

dav1d keeps the tail of a temporal unit buffered once a picture is ready
and rejects new input with EAGAIN until it is drained. Decode() treated
that EAGAIN as fatal, so a trailing OBU would make every later Decode()
fail.

Drain with dav1d_get_picture() until it returns EAGAIN and retry the
send, similar to Chromium's dav1d decoder. A unit without a picture now
returns WEBRTC_VIDEO_CODEC_NO_OUTPUT.

Bug: webrtc:561877904
Change-Id: Ic8545a60f97f29ac4dd1a23c4da8f83b02960792
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/503960
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#48735}