Bug 589371 - some new brushes.

Extract of a selection by Jose Americo Gobbo.
Reviewed by Aryeom and myself for integration.

From what I could gather, authors are:
- Grunge 01 by Rene Jensen.
- Stone Work 01 by Mathias Jonathan (Griffeur).
- Pencil 03 and Charcoal 03 by Americo.

According to information from Americo, all data is GPL v3.0.