Implement GL_NV_read_depth_stencil The implementation will perform two readPixels calls, once for each aspect, and then interleave and pack the result. Bug: angleproject:4688 Change-Id: I46390df893de50b93e855e9333ffab567215a2bb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3702686 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Cody Northrop <cnorthrop@google.com> Auto-Submit: Constantine Shablya <constantine.shablya@collabora.com>