* IsAttachmentEnabled was checking the wrong draw buffer state.  Instead
   of checking that drawbuffer[colorAttachment] is in use, it should have
   been scanning for a drawbuffer state that points to colorAttachment.
 * Allow for maxDrawBuffer != maxColorAttachments.  Tested by the GL
   backend on some systems that don't have the draw buffers extension.

Fixed by updating the helpers and adding a new getDrawBuffer helper.

BUG=angleproject:1121

Change-Id: Idd1b0a9ec4a3f944d332c708364408bf5d59e1fd
Reviewed-on: https://chromium-review.googlesource.com/292740Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>