In _cogl_material_pre_change_notify we need to identify if it's a sparse
property being changed and if so initialize the state group if the given
material isn't currently the authority for it.
Previously we were unconditionally calling
_cogl_material_initialize_state which would e.g. NULL the layer
differences list of a material each time a layer change was notified.
It would also call _cogl_material_initialize_state for non-sparse
properties which should always be valid at this point so the function
has been renamed to _cogl_material_initialize_sparse_state to make this
clearer with a corresponding g_return_if_fail check.
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