[Feature Request] Allow user-defined uniforms that can be set from command line

[Asmageddon]

Example desired usecase:

...
uniform vec3 color_multiplier; // Custom uniform
void main() {
    ...
    gl_FragColor = c * vec4(color_multiplier, 1.0);
}

After which, from the command line I would run e.g. picom --set-uniform "color_multiplier=#ff2222, in order to set the uniform to the value I want it to be set to.

[username24532]

Another thing that might be nice to have is selected window properties being passed as a uniform? Like picom --property-uniforms=saturation,inverted which passes _SATURATION and _INVERTED are numeric properties set by the WM, so it can set effects on specific windows.