Tablet example: smoother, rotation stylus, better airbrush

The drawing is antialiased and with rounded caps and joins.
Rotation stylus acts like a felt marker.
Airbrush sprays a radial gradient of color, and its alpha can
depend on the tangential pressure if so chosen.

Task-number: QTBUG-46694
Change-Id: I4fb16eb621707e3c56a75aa302dd0d3bf418a745
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>