use density instead of number for rays
use ray density for separate texture method
extract KZTexLight
Merge branch 'master' of github.com:kuanyingchou/kz-light2d

Conflicts:
	Assets/kz-light2d/Demos/TestColors.unity
	Assets/kz-light2d/Demos/TestPerlin.unity
	Assets/kz-light2d/Scripts/KZLight.cs