lavfi: add fade filter

Port fade filter from libavfilter soc repo, with minor fixes by
Stefano.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -353,6 +353,39 @@ drawbox
 drawbox=10:20:200:60:red@@0.5"
 @end example
 
+@section fade
+
+Apply fade-in/out effect to input video.
+
+It accepts the parameters:
+@var{type}:@var{start_frame}:@var{nb_frames}
+
+@var{type} specifies if the effect type, can be either "in" for
+fade-in, or "out" for a fade-out effect.
+
+@var{start_frame} specifies the number of the start frame for starting
+to apply the fade effect.
+
+@var{nb_frames} specifies the number of frames for which the fade
+effect has to last. At the end of the fade-in effect the output video
+will have the same intensity as the input video, at the end of the
+fade-out transition the output video will be completely black.
+
+A few usage examples follow, usable too as test scenarios.
+@example
+# fade in first 30 frames of video
+fade=in:0:30
+
+# fade out last 45 frames of a 200-frame video
+fade=out:155:45
+
+# fade in first 25 frames and fade out last 25 frames of a 1000-frame video
+fade=in:0:25, fade=out:975:25
+
+# make first 5 frames black, then fade in from frame 5-24
+fade=in:5:20
+@end example
+
 @section fifo
 
 Buffer input images and send them when they are requested.