webgl crashes tab on 64bit Tor Browser for Windows

On our blog one user noted (https://blog.torproject.org/comment/278911#comment-278911) that https://browserleaks.com/webgl is crashing their tab reliably using 8.5a6 in the 64bit flavor for Windows. This does not happen with the 32bit variant. And neither does this happen with vanilla Firfox 60.4.0esr.

It seems that a bare Firefox compiled our way without any Tor Browser patches is crashing as well. Thus, I wonder whether we hit a GCC bug or something that is only affecting some 64bit system. Mysterious.

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