This cl adds the ability for the ReadPixels function to read other
attachments than the color attachment. Checks were added for both
depth and stencil attachments.

A new test was added (DepthStencilFormatsTest.DepthStencilReadback)
to test this new functionality. As the name mentions, it's used to
test reading from the depth and stencil attachments using ReadPixels.

Bug: angleproject:4295
Change-Id: I6fe9be11f05d6055a5883b4315f870e7c0ac41ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2031702
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>