Add descriptions for the committed filters.

Originally committed as revision 20375 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/vf_crop.c

@@ -214,6 +214,8 @@ static void draw_slice(AVFilterLink *link, int y, int h)
 
 AVFilter avfilter_vf_crop = {
     .name      = "crop",
+    .description = NULL_IF_CONFIG_SMALL("Crop the input video to x:y:width:height."),
+
     .priv_size = sizeof(CropContext),
 
     .query_formats = query_formats,

libavfilter/vf_null.c

@@ -41,6 +41,7 @@ static void end_frame(AVFilterLink *link)
 
 AVFilter avfilter_vf_null = {
     .name      = "null",
+    .description = NULL_IF_CONFIG_SMALL("Pass the source unchanged to the output."),
 
     .priv_size = 0,
 

libavfilter/vf_vflip.c

@@ -87,6 +87,8 @@ static void draw_slice(AVFilterLink *link, int y, int h)
 
 AVFilter avfilter_vf_vflip = {
     .name      = "vflip",
+    .description = NULL_IF_CONFIG_SMALL("Flip the input video vertically."),
+
     .priv_size = sizeof(FlipContext),
 
     .inputs    = (AVFilterPad[]) {{ .name             = "default",