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/+/2216713Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>