xgl:  Add framebuffer-no-attachment support

Add default values if no attachments are present, correctly handle
override values when attachments are present.  Bug# 13212, header
revision #36.
diff --git a/tests/xglrenderframework.cpp b/tests/xglrenderframework.cpp
index 68f7baa..3c452c5 100644
--- a/tests/xglrenderframework.cpp
+++ b/tests/xglrenderframework.cpp
@@ -171,6 +171,9 @@
          .pColorAttachments = m_colorBindings,
          .pDepthStencilAttachment = dsBinding,
          .sampleCount = 1,
+         .width = (XGL_UINT)m_width,
+         .height = (XGL_UINT)m_height,
+         .layers = 1,
     };
     XGL_RENDER_PASS_CREATE_INFO rp_info;
     memset(&rp_info, 0 , sizeof(rp_info));