Blender Git Loki

Keying node: assume overexposured pixels as foreground

Screens are usually doesn't have overexposured pixels and all
saturation / gradient math was written assuming that all channels
are withing 0 .. 1 range and in cases when some channel exceeds
this range matte could be completely wrong.

Added special check for overesposure and assume such pixels as
definitely foreground.

Also fixed minimal value for edge kernel size.