fixed depth buffering for images

resources/examples/vulkan_example.rs

@@ -200,6 +200,8 @@ void main() {
         .viewports_dynamic_scissors_irrelevant(1)
         // See `vertex_shader`.
         .fragment_shader(fs.main_entry_point(), ())
+
+        .depth_stencil_simple_depth()
         // We have to indicate which subpass of which render pass this pipeline is going to be used
         // in. The pipeline will only be usable from this particular subpass.
         .render_pass(Subpass::from(render_pass.clone(), 0).unwrap())