Merge Keying Screen node developed in tomato branch into trunk.
This node is aimed to make dealing with non-even greenscreens better
by generating gradiented image which could be used a input for keyer
nodes.
Based on building voronoi diagram using motion tracking markers as
sites position and average pattern color as color for that site.
Pretty straignforward node, some documentation is there
http://wiki.blender.org/index.php/User:Nazg-gul/Keying#Screen_color
