Several of these methods that deal with the sampler mapping can be
made D3D-only quite simply by casting to ProgramD3D in the Renderer.

BUG=angleproject:1123

Change-Id: Ia6648d26744f9aa69a4a0facb356209df471fa7b
Reviewed-on: https://chromium-review.googlesource.com/293823Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>