<pre style="white-space:pre-wrap;width:81ex">Fix image uploads being random data when canvas read access is blocked (#15180)

Fixes #15178

PR #11499 introduced a way to deal with all-white canvas, but newer
Firefox versions set random data instead.

This PR detects whether canvas operations are reliable by comparing
the results on a hardcoded 2×2 pixels image, and memoizing the result.

This should be both more reliable and faster than the previous check.</pre>
https://github.com/tootsuite/mastodon/commit/d8d43a427a549cf063de3f6b3c22a08a06f53ffa