title: ivec2 is not defined at webgl slider with gl transition
body: <p>I'm using a webgl slider that is using different transitions and you can also use your own transitions. The transitions used are from  <a href="https://gl-transitions.com/" rel="nofollow noreferrer">https://gl-transitions.com/</a> and the one I want to use is this one:
  <a href="https://gl-transitions.com/editor/randomsquares" rel="nofollow noreferrer">https://gl-transitions.com/editor/randomsquares</a></p>
  
  <p>The problem is that I get <strong>ivec2 undefined error</strong>...
  New to webgl...</p>
  
  <p>I've managed implement other effects but I don't understand how to handle ivec2. Is it an array?</p>
  
  <pre><code>const uniforms = {
      size: ivec2(10, 10),
      smoothness: 0.5
  };
  
  const shader = [
      'uniform ivec2 size; // = ivec2(10, 10)',
      'uniform float smoothness; // = 0.5',
  
      'float rand (vec2 co) {',
        'return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);',
      '}',
  
      'vec4 transition(vec2 p) {',
        'float r = rand(floor(vec2(size) * p));',
        'float m = smoothstep(0.0, -smoothness, r - (progress * (1.0 + smoothness)));',
        'return mix(getFromColor(p), getToColor(p), m);',
      '}'
  ].join( '\n' );
  GLSlideshow.addShader( 'myShader', shader, uniforms );
  
  loader.whenReady = function() {
      var slideshow = new GLSlideshow(
          [ 'img/1.jpg', 'img/2.jpg', 'img/3.jpg', 'img/4.jpg' ],
          {
              canvas: document.getElementById( 'myCanvas' ), // optional
              width: 1024,        // optional
              height: 576,        // optional
              duration: 5000,     // optional
              interval: 3000,     // optional
              effect: 'myShader'
          }
      );
  }
  </code></pre>