Not enough test coverage for the canvas compositing operators combined with shadows

[Scony]

As per whatwg/html#10338 one can see, that despite spec being pretty clear on how shadows relate to compositing operators, browser implementations are somewhat random in that regards thus making a fiddle such as https://jsfiddle.net/np1e0uxd/ yielding different results in browsers I tried (no two browsers yield the same results):

• chromium
• firefox
• safari
• epiphany

The above is very likely due to lack of test coverage - there are some tests for compositing operators, but there are not many tests for combinations of compositing operators and draws with shadows - and that's what's pretty tricky.