Option to chose different stroke styles

[vicrucann]

The initial implementation of the stylus sketching is a simple line strip. Each stroke is represented by a set up points, and then it is rendered by using osg::PrimitiveSet::LINE_STRIP.

Under different style of the stroke it is meant:

• varying line width (brush size)
• varying brush type (e.g., pencil, charcoal, brush, etc.)
• varying color and transparency