Blender Git Loki

Fix T42641, Graphical fragments showing on Blender 2.71 and higher when baking.

Safe for 2.73...

This revert rB9b0ab890676790bb1e8e77797629b889ea66f69e - needed to set that threshold to a small
negative value to remove the last artefacts reported in T39735, but now I could not reproduce
any with the previous 0.0f value, so restoring it for the time being.

If this 'shadowed neighbor face' case re-appears, we can always choose a value in-between, like -1e-18f...