primitives: no longer assume _get_layers list remains valid

The internal CoglMaterialLayer pointers associated with a material may
change whenever layer properties are modified so it's no longer ok to
assume that a list of layers returned by cogl_material_get_layers
remains valid if the layers have been changed.

cogl/cogl-primitives.c

@@ -668,6 +668,7 @@ _cogl_rectangles_with_multitexture_coords (
       /* If multitexturing failed or we are drawing with a sliced texture
        * then we only support a single layer so we pluck out the texture
        * from the first material layer... */
+      layers = cogl_material_get_layers (material);
       first_layer = layers->data;
       tex_handle = cogl_material_layer_get_texture (first_layer);