Blender Git Loki

Fix T53371: Keying Node fails with values above 1

This was expected behavior for over-exposured lamps when the mode was originally
created for Tears of Steel. Turns out, there could be really bad green screen in
real production which will only have green (or rather screen) channel over
exposured.

Tweaked condition now so we use least bright channel to see if the area has
proper exposure or not.

Seems to work fine in tests, but further tweaks are possible.