Capture/Replay: Fix readback of zero-sized renderbuffers Only read back the renderbuffer content if the renderbuffer has a non-zero size. Add a test to test the capture/replay in this case. Bug: angleproject:6425 Change-Id: I943804884f18a3a2b9de526309dd500de9dd6fd4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3197572 Commit-Queue: Gert Wollny <gert.wollny@collabora.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>