Only update aspect when there was an actual change.

Fixes playback of e.g. mkv files where SPS/stream and mkv header aspect
information mismatch (or actually even if the SPS/stream aspect value
is simply missing).

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36558 b3059339-0415-0410-9bf9-f77b7e298cf2