When using the debug function _cogl_debug_dump_materials_dot_file to
write a dot file representing the sparse graph of material state we now
only show a link between materials and layers when the material directly
owns that layer reference (i.e. just those referenced in
material->layer_differences) This makes it possible to see when
ancestors of a material are being deferred too for layer state.
For example when looking at the graph if you see that a material has an
n_layers of 3 but there is only a link to 2 layers, then you know you
need to look at it's ancestors to find the last layer.
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