Fix dEQP renderbuffer unspecified attachment test. The spec mandates default sizes of 0 for everything, while we assume a non-initialized Renderbuffer is RGBA4. Bug: angleproject:2321 Test: dEQP-GLES3.functional.state_query.fbo.framebuffer_unspecified_attachment_x_size_rbo Change-Id: I7defbdda46fc90dc3672628667c710a8304473b7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1785881 Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com>