[rectangle] Avoid modifying materials mid scene

[rectangle] Avoid modifying materials mid scene

To improve batching of geometry in the Cogl journal we need to avoid modifying
materials midscene.

Currently cogl_set_source_color and cogl_set_source_texture simply modify a
single shared material. In the future we can improve this so they use a pool
of materials that gets recycled as the journal is flushed, but for now we
give all ClutterRectangles their own private materials for painting with.