Validate GL_COLOR_ATTACHMENT0 separately from the draw buffers attachments. EXT_draw_buffers may not be enabled but the maxColorAttachments cap is always initialized so make sure to validate for the extension instead of just checking that the attachment is in the valid range. BUG=angleproject:2058 Change-Id: I5b48cb496bf96cbc0911295aa5bf87784ce9241b Reviewed-on: https://chromium-review.googlesource.com/735749 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>