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.

0 files changed, 0 insertions, 0 deletions