The D3D-specific code we can split into D3D-only methods, and the
GL-level shared code we can place in Program instead of making Impl
methods. This cleans up the Impl inteface significantly.

BUG=angleproject:1123

Change-Id: Ibcb7d07733eb939adf5bb4f5395a661875a60238
Reviewed-on: https://chromium-review.googlesource.com/293763Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>