We thought we could rely on GLX_create_context_attrib being present but
this is not the case with VM drivers. Instead we fallback to creating
contexts with glXCreateContext that doesn't offer control on which
version and profile the context will have.

Likewise we may not always be able to use the multisample but we can
safely assume that the number of samples and sample buffers are zero in
this case.

BUG=angleproject:1206

Change-Id: I888c69e609764994baf86386c9683b3edeb320f7
Reviewed-on: https://chromium-review.googlesource.com/312230Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>