lavfi: add tile video filter.

doc/filters.texi

@@ -2682,6 +2682,22 @@ Complete example of a thumbnail creation with @command{ffmpeg}:
 ffmpeg -i in.avi -vf thumbnail,scale=300:200 -frames:v 1 out.png
 @end example
 
+@section tile
+
+Tile several successive frames together.
+
+It accepts as argument the tile size (i.e. the number of lines and columns)
+in the form "@var{w}x@var{h}".
+
+For example, produce 8×8 PNG tiles of all keyframes (@option{-skip_frame
+nokey}) in a movie:
+@example
+ffmpeg -skip_frame nokey -i file.avi -vf 'scale=128:72,tile=8x8' -an -vsync 0 keyframes%03d.png
+@end example
+The @option{-vsync 0} is necessary to prevent @command{ffmpeg} from
+duplicating each output frame to accomodate the originally detected frame
+rate.
+
 @section tinterlace
 
 Perform various types of temporal field interlacing.