Fix reference frame size restrictions.

The issue was introduced by commit g9f37d149 with adding explicit
restrictions on reference-frame scale factors. The restriction
is checked against aligned-by-8 frame dimensions, not against
original ones. So, for example, frame of 35×35 actually can refer
to frame of 70×70, but the new check won't allow this. It will
compare 35 vs 72 (not 70), so 2x downscale limit will be exceeded.

Change-Id: Ic663693034440f64ac8312cbff9e1e773a921060