Godot. Very simple mask shader for 2d.

mask.shader

shader_type canvas_item;
uniform sampler2D mask;

void fragment(){
    vec4 col = texture(mask, UV);

    if(col.a == 0.0){
        COLOR = col;
    }
    else if (col.a > .99){
         COLOR = texture(TEXTURE, UV);
    }
    else{
        vec4 col2= texture(TEXTURE, UV);
        col2.a = min(col.a, col2.a);
        COLOR = col2;
    }
}