Godot 1-pixel outline shader

shader_type canvas_item;

uniform vec4 outline_col : hint_color; //Outline color

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

    //Get alpha val of adjacent pixels
    vec2 p = TEXTURE_PIXEL_SIZE;
    float a = texture(TEXTURE,UV+vec2(p.x,0)).a;
    a += texture(TEXTURE,UV+vec2(-p.x,0)).a;
    a += texture(TEXTURE,UV+vec2(0,p.y)).a;
    a += texture(TEXTURE,UV+vec2(0,-p.y)).a;

    //Using found alpha value,
    a = step(a,.5);//Clamp the a value
    col.rgb = mix(outline_col.xyz, col.rgb, col.a);
    col.a = step(a, col.a);

    //Get palette color
    COLOR = col;
}