Re-Add support for PNG or PNM as watermark images, which was broken

since the move to image2 API
Patch by Victor Paesa <wzrlpy@arsystel.com>
Original thread:
Date: Aug 13, 2006 9:18 AM
Subject: [Ffmpeg-devel] [PATCH] Attempt image2 in watermark vhook

Originally committed as revision 6129 to svn://svn.ffmpeg.org/ffmpeg/trunk