Art Brush Options' Width had no pen tablet option, so an art brush
drew at one width whatever the pressure. The Width row now has a
Fixed or Pressure menu and a second width. With Pressure the art is
Width wide at the lightest pen pressure and the second width at the
heaviest, following the pressure the Paintbrush recorded along the
stroke, and half way between the two on a stroke drawn without
pressure. brush.options takes width_mode and width_max; a brush
without them is saved as before.