Some GLX implementations can't handle having libGL.so.1 dlclose'd out
from under them while there's any X window still open against which a
GLXWindow was ever created. Load the library once per process rather
than once per FunctionsGLX instance.

This showed up as a crash in X11Window::destroy, during XCloseDisplay.

BUG=angleproject:892

Change-Id: I0da2674786a952865e5bea2513259c0a32d4453c
Reviewed-on: https://chromium-review.googlesource.com/280902Tested-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@google.com>