Constant GRAYSCALE

const GRAYSCALE: &str = r#"
void main() {
    vec2 uv = gl_FragCoord.xy / u_res;
    vec4 src = texture(tex, uv);
    float l = dot(src.rgb, vec3(0.2126, 0.7152, 0.0722));
    vec4 res = vec4(mix(src.rgb, vec3(l), clamp(p0, 0.0, 1.0)), src.a);
    float m = u_has_mask > 0.5 ? texture(u_mask, uv).r : 1.0;
    out_color = mix(src, res, m);
}
"#;