OSX: Investigate whether offscreen rendering must run on _the_ main thread

[phronmophobic]

No description provided.

[phronmophobic]

It doesn't seem like there's an official doc or response that covers this. See:

• https://www.magpcss.org/ceforum/viewtopic.php?f=10&t=18273&sid=faa44791961276d879788d95d258c6d9
• https://bitbucket.org/chromiumembedded/cef/wiki/GeneralUsage#markdown-header-threads
• https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html

[phronmophobic]

Initial tests haven't crashed.

[phronmophobic]

I've found a few cases of spurious crashes. I think the crashes are attributable to pop ups automatically being given their own window by cef. Trying the following work around:

(cef/map->life-span-handler
            {:on-before-popup
             (fn [& args]
               ;; cancel popups
               1)
...
})