Add EXT_separate_depth_stencil stubs Bug: angleproject:362780331 Change-Id: I378b5c9ff06a2c5e56d68ef93ed42e0351b5044e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5823195 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>