721744f2

By: Michael Lynch <git@mtlynch.io>

Preserve Tinybeans GIF timing in AVIF imports

The previous ffmpeg invocation let the encoder resample GIF frame timing,
which collapsed source delays like 40ms and 50ms into a synthetic constant
rate. Imported animated AVIFs then played at the wrong speed.

Add fps_mode passthrough and a centisecond encoder time base so ffmpeg keeps
Tinybeans GIF timing intact when writing animated AVIFs. Also add a unit test
that locks in the ffmpeg argument list.

Timeline