vp9: disable use_last_frame_mvs on resolution change (scalable).

Prevents some invalid memory accesses after resolution change in
vp90-2-05-resize.webm, and libvpx does this too.

Reviewed-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vp9: fix invalid ref frame w/h on size change.

Fixes invalid reads and crashes in vp90-2-05-resize.webm and fuzzed6.ivf.
The output is still not identical to what libvpx does (because we don't
actually scale in MC).

Reviewed-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>