The first step for true multithreading support is allowing multiple
threads to use ANGLE as long as only one thread has a current context
at a time.
Added a test to MultithreadingTests to reproduce this behavior and
implemented it in GLX.

Bug: angleproject:4724
Bug: angleproject:4725
Bug: chromium:1087084
Change-Id: I908d3f02f34a681f1c9d0919dd17aee1a489173c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2241616
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>