Add coverage of copying a canvas drawing_buffer after the end of the RAF

[Kangz]

Chromium had a longstanding bug that it couldn't make a copy of a drawing buffer after it had been sent to the compositor. This was fixed recently but no new tests started passing, which indicates missing coverage.

We should have tests for transferToImageBitmap, toBlob, toDataURL and friends that look like:

requestAnimationFrame( () => {
   do the drawing
})
// wait for the animation frame
call toBlob and friends here