v.1.8.0

• Implement screen-door transparency as an alternative to alpha-based transparency. To enable screen-door transparency, pass transparency : 'screendoor' when constructing the viewer.
• Added special selection highlighting render mode.
• Improve handling of click event by only firing click events when the mouse button is pressed/released within a short timespan.
• Improve rendering of outline by properly scaling the extrusion factor based on the size of the GL canvas.
• Added more sample code to documentation