fix notification dot

What does this MR do?

fix notification dot

1. the crossOrigin='anonymous' tells the browser to actually honour CORS on the image

2. the dot is now always 1/5 of the icon, even for large icons

3. we use toBlob instead of toDataURL because https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL says so

4. the test now blinks the dot, so it tests the actual code

5. if there's a problem during the test, we log the error, so people can tell it to us

Thanks to Ares on Discord for reporting the problems.

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