Add: allow loading heightmaps from command-line (#11870)

If you want to load a file from tar, you have to give the file
inside the tar in order for it to work:

<tar-file>/<dir-in-tar>/<file>.png

Codechange: pass "ground vehicle" to GetTileSlopeZ since for tunnel/bridges there are two states

Previously it checked the position in non-driving direction to "guess" whether
a ground vehicle was using the function, so on tunnels/bridges it could either
return the Z of the (virtual) ground compared to the Z of the path the vehicle
would take.

(svn r11947) -Feature: Make use of new sprites added by Action5 type 0D.
Tiles which only consist of shore do not flood anymore, instead they get removed if they are no longer connected to flooding water.

(svn r9371) -Feature: Add support for variable snow lines in the arctic climate, supplied
by newgrf files. When this is enabled forests cannot be built below the highest
snow line, and farms can't be built above it. Houses still use the
_opt.snow_line so they are all consistent, so to make them respect the snowline
you may want to use some newhouses features as well.