Cycles: mist pass added, with start/depth/falloff control. If the pass is
enabled in a render layer a Mist Pass panel will be shown in the world
properties.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57278 954f8c5b-7b00-dc11-b283-0030488c597c