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/+/1785881Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>