Fix image uploads being perfectly white when canvas read access is blocked

[ClearlyClaire]

Fixes #11496

Note that some versions of the Tor Browser fail earlier in loadImage, but I couldn't fix it.

[Gargron]

It's not impossible that someone might upload a totally white image at some point. I think reading all the pixels one more time is also not great. I suggest using a "canary" check instead. Like try creating a hardcoded all-black, small canvas and reading from it, then check if all the pixels are white. This can also be done once per page load to confirm whether the canvas method is available, instead of on each image upload.

[ClearlyClaire]

This will stop at the first non-255 component, which is very likely to occur very fast except for some specific computer graphics. Also, this check is performed on the resized image, which is at most 1280×1280 in size.

It is true that someone may want to upload a totally white image, in which case it will upload it, just the non-resized version of it…

Doing it just once per page instead of more might make sense, but also, this means the resizing code wouldn't be called ever again even if the user accepts canvas access (as on Tor Browser and Firefox with appropriate privacy settings, it will return an all-white image as long as the user hasn't accepted it—the user prompt does not block the call)