Request average_fill ported from slang to glsl

[riggles1]

Average Fill fills out black borders with an average color of all pixels in the area next to the bar (you select the number of lines that get sampled)
https://github.com/libretro/slang-shaders/tree/master/border
The result can be an extended sky color or ground color instead of just black borders.

One of the usage platforms for it is a Pi4 CRT TV (RGB Scart hat) build that is optimized for gl.