Create a DefaultAttachment type and an implementation for it. This allows for dynamically sized default attachments instead of calling Context::makeCurrent each time the surface changes size. BUG=angle:824 Change-Id: Ic39c0d7dc4269db53a34c01c4d915cb1a3cfbd08 Reviewed-on: https://chromium-review.googlesource.com/228180 Tested-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>