GL: Check for VAO support before syncing from external ctxs Reading and writing VAO state was done unconditionally when syncing from external contexts. This caused crashes when trying to call null VAO-related function pointers. Bug: chromium:1447456 Change-Id: I866dd75a3f66d39d19c7da748d64a00ce02db5e6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4562937 Reviewed-by: Peng Huang <penghuang@chromium.org> Reviewed-by: Colin Blundell <blundell@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>