Fix bugs with drawbuffer state. * 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/292740 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tryjob-Request: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>