In the middle of a downtown Oakland studio, two artists stand on opposite sides of a canvas with brushes at the ready. They have no prompt, only 20 minutes to paint something. Music blares as the minutes tick down. Zoë Boston and Bud Snow are first up, and dozens of people begin to orbit the artists as their brushes hit the canvas.  They went through the drill, as did a host of other artists, to raise money for a local…