avviddec: Fix pool reallocation logic

avviddec: Fix pool reallocation logic

Some check where incorect and also unsafe. The only reliable information
in get_buffer2 is the picture width/height really. The side effect is
that the width/height of the internal pool endup padded, so when we
switch we also need to switch to the a new width/height, hence we save
the pool info.

https://bugzilla.gnome.org/show_bug.cgi?id=753869