make the tube connection images for furnaces/chests into overlays
also don't redefine furnace top - tubes don't connect there anyways
add "metallic" tube connection image for other mods to use
catch nil return from facedir_to_dir() in tube autoplace code
Revert "catch nil return from facedir_to_dir() in tube autoplace code"

This reverts commit 4f9c7a43c9f1f0ab3286c755c49bd8d2951b4dc8.
Better way to catch that incoming nil, return a real value on the way out
More sanity checks on the nodeside() function, courtesy ShadowNinja
catch a similar nil condition in the node breaker