material: Adds _cogl_material_pre_paint_for_layer

material: Adds _cogl_material_pre_paint_for_layer

This is a counter part for _cogl_material_layer_pre_paint which takes a
layer index instead of a direct CoglMaterialLayer pointer. The aim is to
phase out code that directly iterates the internal layer pointers of a
material since the layer pointers can change if any property of any
layer is changed making direct layer pointers very fragile.