Ensure that hints are drawn within viewport

[aadcg]

While reviewing #3226, I noticed we don't provide this guarantee. In the example below, https://en.wikipedia.org/wiki/List_of_municipalities_in_Quebec has been zoomed as to have an element on the right edge.

[aartaka]

This is not the problem with hint system, it's a bug in foreign interface. So here's how it happens:

• Invoking zooming commands modifies buffer zoom settings.
• This setting modification triggers (setf ffi-buffer-zoom-level) method.
• This method tries to dispatch over the current buffer.
  • If it's a regular buffer, use fallback JavaScript-based zooming.
  • If it's a gtk-buffer, use WebKit-provided scrolling.
• Here's when it breaks: buffer is inherited from gtk-buffer and thus is more specific.
• So JavaScript-invoking method is picked instead of FFI one.
• And thus the zooming is slightly broken, because JS-based zooming is somewhat broken by design.

[aartaka]

Or we have two bugs 😃

[aadcg]

The discussion about zooming is orthogonal to the issue at hand, as demonstrated in the screenshot below at URL https://maximilian-schillinger.de/tmp/hint-test.html.

[aartaka]

The discussion about zooming is orthogonal to the issue at hand, as demonstrated in the screenshot below at URL

Okay, then it was right for me to open a separate issue.