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/1337453Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>