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.

src/content_mapblock.cpp

@@ -276,7 +276,7 @@ void MapblockMeshGenerator::drawCuboid(const aabb3f &box,
 void MapblockMeshGenerator::getSmoothLightFrame()
 {
 	for (int k = 0; k < 8; ++k) {
-		u16 light = getSmoothLight(blockpos_nodes + p, light_dirs[k], data);
+		u16 light = getSmoothLightTransparent(blockpos_nodes + p, light_dirs[k], data);
 		frame.lightsA[k] = light & 0xff;
 		frame.lightsB[k] = light >> 8;
 	}