anchor.diff:                    cmd-buf :vertex
                     c (- s) s c))
                ;; Draw
  -               (virgl:add-command-draw-vbo cmd-buf 0 6 :triangles :indexed t)
  +               (virgl:add-command-draw-vbo cmd-buf 0 (length test-indices) :triangles :indexed t)
                ;; Do it!
                  (virgl:command-buffer-finalize cmd-buf)
                  (virgl:command-buffer-submit cmd-buf)
  
positive.code:           ;; Enable writes to the color channels
            (virgl:add-command-bind-blend setup-cmd-buf blend)
            (virgl:add-command-bind-rasterizer setup-cmd-buf rasterizer)
            (virgl:add-command-set-index-buffer setup-cmd-buf index-buffer 2 0)
            (virgl:command-buffer-finalize setup-cmd-buf)
            (virgl:command-buffer-submit setup-cmd-buf)
  
negative.code:                (blend (virgl:make-blend context :colormask :rgba))
                 (rasterizer (virgl:make-rasterizer
                              context
                              :front-ccw t ; match opengl
                              :cull-face :back))
                 (dsa (virgl:make-dsa
                       context
                       :depth-enabled t
                       :depth-writemask t
                       :depth-func :less))
                 (setup-cmd-buf (virgl:make-command-buffer context)))
            ;; Attach scanout surface object to the framebuffer's color0 channel.
            (virgl:add-command-set-framebuffer-state setup-cmd-buf depth-surface scanout-surface)
            ;; Viewport.
            (virgl:add-command-set-viewport-state
             setup-cmd-buf
  