This is WebKit bug https://bugs.webkit.org/show_bug.cgi?id=46319

The workaround is to wrap Canvas render functions with a function that
sets a flush timer. The flush function sets the right margin and then
1ms later sets it back. This triggers the canvas to redraw with the
correct contents.

Two downsides:
- rendering is slower, but only on the busted versions of webkit.
  Correct and useful is better than fast and useless.
- There is a barely perceptible jitter of the control buttons because
  the canvas size is changing by one pixel.

To support this functionality, we also have to read out the exact
webkit version from the user agent in the render engine detection code
in include/util.js.