We can use a faster path to write out depth values. Previous attempts
were foiled by a D3D quirk: we need to clear a new DSV before we
write to it the first time.

Multisampled stencil resolve still requires a CPU readback.

BUG=angleproject:1710

Change-Id: I7a7d2f929644f61854d51dd096a8aa599ad648bd
Reviewed-on: https://chromium-review.googlesource.com/435571Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>