The `anim` data (e.g. of a VSE sequence) would store the last frame read from
FFMPEG as an image buffer, increasing its refcounting and preventing it
to be freed until the whole `anim` data itself gets freed.
In current code, there is no reason to keep a reference to this image
buffer in the `anim` data, so removing it. This may also give a few
percent improvement on the memory usage of the VSE in some cases.
Found while investigating #114342.
Bastien Montagne