Add sws_getCachedContext(), which checks if context is valid or reallocs a new one instead.

Patch by Victor Paesa <wzrlpy@arsystel.com>
Original thread:
Date: Aug 31, 2006 7:15 PM
Subject: [Ffmpeg-devel] [PATCH] Add sws_getCachedContext() to swscale library

Originally committed as revision 19667 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale