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/228180Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>