This patch introduces a new Framebuffer::Data class, which stores
attachment related state. This will eliminate the need to store
duplicated state between the classes.

BUG=angleproject:930

Change-Id: I80de4db39ab99d623b0ad8306bf3cbb794cd8bd5
Reviewed-on: https://chromium-review.googlesource.com/254100Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>