Allow querying of all draw buffer states on the default framebuffer. Because the default framebuffer isn't tied to a specific context, its drawbuffer state vector was only of size 1. It is still valid to query up to GL_MAX_DRAW_BUFFERS through so a special case is added when the drawbuffer is larger than the state vector. BUG=angleproject:2965 Change-Id: Ib49570df67e59e93932a7e916fe72f2e71c29939 Reviewed-on: https://chromium-review.googlesource.com/c/1337453 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>