material: tweak dot file debug code
material: tweak dot file debug code

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.

jigpu/clutter,jigpu/clutter,ebassi/clutter,djdeath/clutter-multithreaded,spatulasnout/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,dlespiau/clutter,Distrotech/clutter,dlespiau/clutter,Distrotech/clutter,heysion/clutter-clone,Distrotech/clutter,collects/clutter,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter,ebassi/clutter,kerrickstaley/clutter-vala,djdeath/clutter,ebassi/clutter,nobled/clutter,djdeath/clutter,collects/clutter,djdeath/clutter,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,ebassi/clutter,djdeath/clutter-android,spatulasnout/clutter,jigpu/clutter,spatulasnout/clutter,jigpu/clutter,nobled/clutter,ebassi/clutter,GNOME/clutter,djdeath/clutter-android,GNOME/clutter,djdeath/clutter-android,jigpu/clutter,collects/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,collects/clutter,djdeath/clutter-android,jigpu/clutter,nobled/clutter,collects/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,djdeath/clutter,spatulasnout/clutter,Distrotech/clutter,GNOME/clutter,heysion/clutter-clone,heysion/clutter-clone,GNOME/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,GNOME/clutter,dlespiau/clutter