Fixed expected state when using different images for depth and stencil

When depth and stencil are both present, they must be the same image,
or return GL_FRAMEBUFFER_UNSUPPORTED, according to the GLES 3.0 spec,
section 4.4.4, Framebuffer Completeness.

Change-Id: I607c05617fb55743635eda987da6de523385d92b
Reviewed-on: https://swiftshader-review.googlesource.com/5262
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>