title: tf.browser.fromPixels returns only zeros
body: <p>Here tf.browser.fromPixels returns a tensor with only zeros. image.data contains values as expected (not only zeros). tfjs is loaded using a script tag. tfjs 1.0.0 but changing release does not help. </p>
  
  <p>What can be wrong? Probably something stupid. I am new to Tensorflow.js... </p>
  
  <pre><code>&lt;canvas id="canvas" width="100" height="100" style="border:1px solid #FF0000;"&gt;&lt;/canvas&gt;
  ....
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');   
  var image = ctx.getImageData(0, 0, canvas.height, canvas.width);
  var x = tf.browser.fromPixels(image);
  </code></pre>