Don't reallocate default FBO on makeCurrent. We can use the flat FBO attachment types to update our data instead of reallocating it. This also lets us delete the DefaultFramebuffer class. BUG=angleproject:963,angleproject:840 Change-Id: Ib8f20d8212d073fb5e248756321cb2e6b4e086dc Reviewed-on: https://chromium-review.googlesource.com/263492 Reviewed-by: Kenneth Russell <kbr@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>