Feature Request: Dimming unselected region.

[Momijie]

Could we dim the unselected regions for slop? Most selection tools use that to make it clear what's being selected and what isn't.
Any interest in adding a feature like this to the project, if so what would be a good approach, I'm interested in getting something like this implemented to greater standardize the user experience of selection on my system.

If an example is needed, think of the selection process of flameshot.

[HumanoidSandvichDispenser]

Here's a glsl fragment shader that dims the unselected region

#version 120

uniform sampler2D texture;
uniform sampler2D desktop;

varying vec2 uvCoord;

void main()
{
    vec2 uv = vec2(uvCoord.x, -uvCoord.y);
    vec4 color = texture2D(desktop, uv);
    vec4 rect = texture2D(texture, uvCoord);
    gl_FragColor = vec4(vec3(0.0), color.a * 0.5) * (1.0 - rect);
}

Run slop with -l so it doesn't dim inside of the rectangle

There's also a shader that does the same thing but can also draw selection borders by calculating the size of a pixel in UV coordinates: #146 (comment)