TK: Individual tileOverlap per Layer

When using a special layer with only black, half transparent tiles (e.g. to darken specific tiles with a fog of war), using the TKMap.tileOverlap = 0.25 setting re-introduces the dreaded black lines, since on the junctions in between the tiles the fog-of-war-tile gets drawn twice.

If this option could be switched on on a per layer basis, one could profit from it for the solid ground tiles, but ignore it for layers with half-transparent tiles. Not sure, if this would improve the rendering, though, as this may introduce other problems.