Slope tools with no zero-height corners

[Leartin]

I noticed that the slope tool configuration in the toolbars supports not only 'straight' slopes, but all kinds of slopes. So I made a menu with every possible slope. That works for all slopes which have at least one corner at zero height. Those which have no zero-height-corners can be used and create correct slope walls, but no correct "roof".

I guess this is because slopes without at least one zero-height corner do not really exist, and it was probably never planned to have anything but straight slopes in the toolbars anyway, since they are not that useful. Still, I wanted to report it in case it's easy to fix, or to get an 'official' statement that it's not intended.

[Dwachs]

This is intended (in the sense that the terraform code cannot handle slopes with no zero-height corners besides all-up slopes).