Fix image uploads being perfectly white when canvas read access is blocked (#11499)

Fixes #11496

app/javascript/mastodon/utils/resize_image.js

@@ -67,6 +67,14 @@ const processImage = (img, { width, height, orientation, type = 'image/png' }) =
 
   context.drawImage(img, 0, 0, width, height);
 
+  // The Tor Browser and maybe other browsers may prevent reading from canvas
+  // and return an all-white image instead. Assume reading failed if the resized
+  // image is perfectly white.
+  const imageData = context.getImageData(0, 0, width, height);
+  if (imageData.every(value => value === 255)) {
+    throw 'Failed to read from canvas';
+  }
+
   canvas.toBlob(resolve, type);
 });