doc/filters: add geq diagonal split screen example

Also remove similar but confusing and less useful example.

Based on geq expression by Patrick Race <raceink at gmail>.
Signed-off-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -7478,12 +7478,9 @@ geq=lum=255*gauss((X/W-0.5)*3)*gauss((Y/H-0.5)*3)/gauss(0)/gauss(0),format=gray
 @end example
 
 @item
-Create a linear gradient to use as a mask for another filter, then
-compose with @ref{overlay}. In this example the video will gradually
-become more blurry from the top to the bottom of the y-axis as defined
-by the linear gradient:
+Diagonal split screen to compare filter effect:
 @example
-ffmpeg -i input.mp4 -filter_complex "geq=lum=255*(Y/H),format=gray[grad];[0:v]boxblur=4[blur];[blur][grad]alphamerge[alpha];[0:v][alpha]overlay" output.mp4
+ffmpeg -i input -filter_complex "[0:v]geq=lum=if(gt(X\,Y*(W/H))\,255),format=gray[alpha];[0:v][alpha]alphamerge,curves=preset=color_negative[filtered];[0:v][filtered]overlay" output
 @end example
 @end itemize