Sort video filter descriptions by name.

Originally committed as revision 20344 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/libavfilter.texi

@@ -80,10 +80,6 @@ build.
 
 Below is a description of the currently available video filters.
 
-@section null
-
-Pass the source unchanged to the output.
-
 @section crop
 
 Crop the input video to x:y:width:height.
@@ -115,4 +111,8 @@ corner of the input image.
 
 The default value of ``w'' and ``h'' is 0.
 
+@section null
+
+Pass the source unchanged to the output.
+
 @bye