Fix race condition in Whirly.stop

[Thalagyrt]

Hiya! I've run across this one a few times using this, there's a race condition in the stop method that can cause unrender to throw an exception.

I see there's two failing tests in this PR, but they are failing on master as well.

• Thread#terminate is not synchronous, it only schedules the thread
  to be killed.
• The thread in the background can on rare occasions unset
  @current_frame after line 1 of Whirly.unrender, causing
  a nil to be passed to Unicode::DisplayWidth.of, and causing
  an exception as a result.
• When we kill the thread, we need to join it to wait for it to
  finish before proceeding, otherwise we risk memory corruption.

[Thalagyrt]

Just a note, I'm only 99.9% sure on this fix. It's exceedingly rare that the race condition hits, but all the symptoms line up with a thread race condition given how Thread#terminate works. I'll definitely have more info in about a week after people have had some time with it, we use this internally a lot and someone will for sure run into it and let me know if it's still happening sooner or later.

Stack trace for posterity:

Traceback (most recent call last):
<snip internal stuff>
         2: from /usr/local/bundle/gems/whirly-0.2.6/lib/whirly.rb:187:in `stop'
         1: from /usr/local/bundle/gems/whirly-0.2.6/lib/whirly.rb:208:in `unrender'
/usr/local/bundle/gems/unicode-display_width-1.4.1/lib/unicode/display_width.rb:9:in `of': undefined method `codepoints' for nil:NilClass (NoMethodError)

This specific exception hits if you call Unicode::DisplayWidth.of(nil), which led me to look at the threading implementation, since Whirly.unrender at a glance looks completely safe from passing a nil, unless a thread modifies the instance in parallel.

Re: Thread#terminate, this behavior is demonstrated here:

irb(main):001:0> t = Thread.new { sleep 10 }; t.terminate; puts t.inspect
#<Thread:0x00007fbdf00c6e70@(irb):1 run>
=> nil
irb(main):002:0> t = Thread.new { sleep 10 }; t.terminate; t.join; puts t.inspect
#<Thread:0x00007fbdee814d78@(irb):2 dead>
=> nil