modified_shader.glsl

#include <flutter/runtime_effect.glsl>

uniform vec2 uSize;
uniform float iTime;
vec2 iResolution;

out vec4 fragColor;

#define PI 3.1415926535897932384626433832795

void main(void) {

    iResolution = uSize;
    vec2 fragCoord = FlutterFragCoord();
    
    // ...
    // ...
}