This is so we can use a different visual depth than our parent window so
that we can create a subwindow whichever the parent's visual.

BUG=angleproject:892

Change-Id: I9bab676761b1f80fad1599adb8e97a3ec710f67f
Reviewed-on: https://chromium-review.googlesource.com/271425Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>