[actor] Avoid modifying materials mid-scene to improve journal batching

[actor] Avoid modifying materials mid-scene to improve journal batching

Currently cogl_set_source_color uses a single shared material which means
each actor that uses it causes the journal to flush if the color changes.
Until we improve cogl_set_source_color to use a pool of materials that can
be recycled as the journal is flushed we avoid mid-scene material changes by
giving all actors a private material instead.