Similar to the Framebuffer and other classes, this gives the Impl
class a read-only view of the object's state.

BUG=angleproject:1123

Change-Id: I580eaebe2de236adf8131d6e3f54633cecce8c25
Reviewed-on: https://chromium-review.googlesource.com/293760Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>