Fix client-side resizing of image uploads (#11570)

Fixes #11567

app/javascript/mastodon/utils/resize_image.js

@@ -71,7 +71,7 @@ const processImage = (img, { width, height, orientation, type = 'image/png' }) =
   // 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)) {
+  if (imageData.data.every(value => value === 255)) {
     throw 'Failed to read from canvas';
   }