color option for perspective filter

[Michael Koch]

Summary of the bug:
When the "perspective" filter is used with parameter "sense=1", the size of the video becomes smaller and the outer area if filled with the pixel values from the border of the input video. It would be nice to have an option for filling the outer area with a color.

This example shows the problem:

rem Make a chessboard video
c:\ffmpeg\ffmpeg -f lavfi -i color=black:s=vga -vf geq=g='255*mod(floor(X/40)+floor(Y/40),2)' -t 5 -y in.mp4

rem Apply the perspective filter
c:\ffmpeg\ffmpeg -i in.mp4 -lavfi "perspective=x0=150:y0=125:x1=500:y1=75:x2=100:y2=375:x3=480:y3=420:sense=1" -y out.mp4

A known workaround is to pad the input video with a 1-pixel wide border (with alpha=0), before applying the perspective filter.
This is useful if you have a video in which a flickering computer screen is visible, and you want to overlay another video over this computer screen.

[Michael Koch]

See also ticket 10280:
​https://trac.ffmpeg.org/ticket/10280