Support user defined shaders

[coreymwamba]

This is more of an feature request: when I used to use i3 I also used picom's legacy glsl shaders to turn my screen from RGB to grayscale, or invert the colours. Having looked at the source code for scenefx (after trying—and failing—to write a script that could modify gamma ramps in wlroots) I see that it uses gles fragment shaders.

So would it be possible to add that functionality to scenefx (and thus swayfx)?