Add support for NV_read_depth, NV_read_stencil and NV_depth_buffer_float2 extensions 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>