Demonstrates how to use IconPainter.
IconPainter can efficiently render large numbers of icons in fixed
locations in data space by leveraging a TextureAtlas to store the icon images.
Picking support is also provided, allowing the IconPainter to report on
the identity of icons under the current mouse location.
Tested with 100,000 individual icons on Nvidia GTX 480. Less with a large number
of icon groups.