DisplayGLX: validate configs against the backing context's config This is a step required before adding an ANGLE extension that will allow passing the Visual ID to use for the context at display creation time: the configs will have to be compatible with it even if the visual has non-standard sizes. BUG=522149 Change-Id: I85e8ee3b9072934c665bddfa2b461d26f8903ff3 Reviewed-on: https://chromium-review.googlesource.com/311220 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>