Add: Dedicated server heightmap support

[Berbe]

Motivation / Problem

Heightmap maps aren't loadable by dedicated servers.
Furthermore, once such a map is loaded, on game end a normal, procedurally generated map takes turn.

Description

Somehow related to #7285 & #7286.
Add heightmap maps support for dedicated servers:

• CLI switch
• Heightmap map reload on game end

Limitations

Works wonders for my needs.
As noted long ago, there might be unoticed side-effects for others game modes (client ,editor), for which your expertise would be valuable.

Those changes have been adapted to the partial network code rework operated in the latest major version.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[2TallTyler]

The target for PRs should always be master, not a release branch. I'm not sure what happened when I switched it back -- can you please rebase atop master and force-push? That should also trigger the CI checks. Sorry! 🙂

[PeterN]

@Berbe Are you able to update this PR and unmangle it?

[Berbe]

@PeterN Sure: w/o bragging, I'm a savvy rebaser, but… I am not sure I'm following what was attempted here. What a mess!
@2TallTyler Which commit shall be salvaged and rebased on master?

[PeterN]

Can you just force push your branch again at least?

[Berbe]

Ah.
I see the original target was the release/13 because that is what I use on top of the current release.

Now you want it on master.
The code I submitted was prepped for v13, but you rewrote stuff on master which requires some rewrite, part of it non-trivial.
This is more work than a simple branch switch rebase. Now I suddenly need to produce code for v14+…

[Berbe]

Replaced with #11386