Add headlights, upgrade PropertyTools package
Coerce number of divisions for RectangleVisual3D
Add CreateMaterial overload that takes brightness of specular color as parameter
Add more options to headlight demo
Merge remote-tracking branch 'upstream/master' into features/headlights
Merge branch 'features/headlights' of https://github.com/objorke/helix-toolkit into objorke-features/headlights
Merge branch 'objorke-features/headlights'