GLES2: Use require_constant_initialization for g_Mutex A static assert to verify that the global mutex g_Mutex is trivially constructed fails to compile with clang when using the STL shipped with Visual Studio 2019. Use __attribute__((require_constant_initialization)) instead to verify for constant initialization. BUG=angleproject:3936 Change-Id: I5969762ad5a99033143513d7c4992344da276b1a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1832164 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>