D3D11: Implement EXT_conservative_depth Drive-by: * Hid the extension from ES 2.0 client contexts. Bug: angleproject:8046 Change-Id: I8bca4161dde4bda7ee75541b9164777884900d89 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4366784 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>