Adds first step to allowing multithreaded contexts: letting multiple threads use ANGLE as long as only one thread has a current context at a time. Bug: angleproject:4724 Bug: angleproject:4725 Bug: chromium:1087084 Change-Id: Ia6ca48c5fa838b93e49fc9ea259d626029439ca3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2257273 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>
