RV20: try keeping aspect during resolution changes.

RV20: try keeping aspect during resolution changes.

Resolution changes are usually only used to scale with
network bandwidth, the (full) resolution specified in the
RM header really is authoritative.
While I am not sure this is the best solution, it is a
conservative approach that still should fix the most
common cases.
In particular, it fixes aspect with the sample from trac
issue #785 (in MPlayer, ffplay seems to just ignore
sample aspect changes in mid-playback).

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>