features/units rendered when making hidden tiles transparent

[chdoc]

When toggling TOGGLE_SHADE_HIDDEN_TILES (b key), stonesense still renders some information on the hidden tiles. In the screenshot, this even includes a forgotten beast that is not listed in the units tab.

Also, in the case of cavern layers, large columns and large open spaces are virtually indistinguishable. It may be reasonable to render the bottom-most layer of solid regions as a black floor to distinguish them from large open "pits", preferably as an optional toggle.