Smooth lighting: Fix light leaking through edge-connected corners

Smooth lighting: Fix light leaking through edge-connected corners

For solid nodes, the lighting at a corner becomes face-dependent, which
means that only the four nodes in face-direction contribute to the
lighting (getSmoothLightSolid).
For special nodes, which use the lighting values at the eight corners of
a node, the lighting is not face-dependent, but certain nodes of the
eight surrounding nodes of a corner (here indices 4, 5, 6 and 7) can be
obstructed.

Ambient occlusion now also occurs for solid nodes, if two, three or four
of the four nodes in face-direction are solid.