avif-decoder: Send pending data to dav1d when Err(Again)

[metamuffin]

image-rs currently errors with Try Again (returned from get_picture) when decoding AVIF images.
This is because dav1d needs to be explicitly flushed via send_data to ensure the stream end is processed.
For reference see dav1d-rs "example" usage
This PR adds a call to send_pending_data when required to do exactly that, fixing the error.

This PR resolves #1954.

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

[fintelia]

Should the Error::Again case be handled in a loop? The example seems to repeatedly send data when it gets an error, rather than only doing so once

[metamuffin]

It appears that for single-frame videos where all data is sent at once this is acceptable.