Handle non-fatal error dav1d::Error::Again

[FlareFlo]

Issue location:
src/codecs/avif/decoder.rs line 43 and 50

Currently, when send_data is called, its non-fatal error variant Again is treated as a fatal one, where this may be treated by send_pending_data, as mentioned by the dav1d-rs docs (dav1d-rs docs fail to build so i used this screenshot from my local build).

#2023 addresses this issue as well, but does not handle the case of Alpha channels and only calls send_pending_data once.

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to choose either at their option.

[fintelia]

Thanks!