Passability Overlay

Adds a new overlay to display passability information. The game uses a grid of tiles to know where are the obstacles. These tiles are marked as impassable and the units cannot traverse them. A unit can find a path only if there are adjacent passable tiles (thus if only diagonal tiles are passable, the path cannot go through). The passability overlay uses a color coding to represent that: Red tiles on the terrain are impassable to all. Yellow tiles are impassable to large units (like mechs, ulfens, etc), but passable to small units (like the survivors). Green tiles on the floors are passable to all. Tiles without color are passable on the terrain, but impassable on the floors. The color coding has been chosen in that specific way so that its computation time and the memory consumption are minimum. The overlay has a shortcut Ctrl-9. It can be used inside the map editor as well, as the overlay buttons are not available there. Updates: Added mod options allowing the overlay to display build levels and voxel grid. Also an option is added to hid the grass when the overlay is active, to improve readability.