simplifies null vhook by using sws_getCachedContext()

Patch by Victor Paesa %wzrlpy A arsystel P com%
Original thread:
Date: Sep 15, 2006 6:21 PM
Subject: Re: [Ffmpeg-devel] [PATCH] enable libswscale

Originally committed as revision 6651 to svn://svn.ffmpeg.org/ffmpeg/trunk