Don't allow glReadPixels of depth buffers with ES3.1. The ES3.1 spec does not appear to include these formats as valid glReadPixels formats. Test=KHR-GLES3.packed_pixels.rectangle.depth24_stencil8 BUG=angleproject:3520 Change-Id: I6ccd5a7fc49c2e16885cfaace56efce1d96e937a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2216713 Reviewed-by: Alexis Hétu <sugoi@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com>