i.e., not only the mod with missing dependencies, but also the missing
dependencies itself. (This already used to be the case before the mod
selection gui was added)

Also, mods with unsatisfied dependencies are no longer reported as
mods that could not be found.

Remove use of operator[] on a std::map, so no spurious elements get inse...

This way, plants actually show the real backface on their back side,
i.e., the front face mirrored around the vertical axis, instead of
showing the front face on both sides. This looked weird when the
texture was not symmetrical around the vertical axis.