[Backrooms] Update to use indestructible terrain, whitelist and EXTERNAL_OPTIONs

[Procyonae]

Summary

None

Purpose of change

Fixes #67321 (for backrooms, underlying C++ function could do with changing to one weighted int list entry per "default_oter" or something similarly moddable)

Describe the solution

Adds an indestructible border terrain for non z0 zlevels and replaces the normal layers with it
Makes floor terrains indestructible
Adds a whitelist
Adds EXTERNAL_OPTIONs to disable normal mapgen
Adds "extend" and "delete" behaviour for mapgen_extra's "flags"
Adds copy-from support for mapgen_extra's "color"
Adds connects_to to the floor terrains so the terrains with lights multitile with the non light versions

Describe alternatives you've considered

Testing

Game compiles and loads
Checked all map extras spawn as expected
Checked bashing terrains works as expected
Checked floor terrains multitile as expected

Additional context

[Procyonae]

@onura46 I'm adding you as a "reviewer" for Backrooms which means it'll ping you if someone has an open (non-draft) PR that touches the Backroom mod files because you're its maintainer, if you don't want that I'll remove it tho

[onura46]

@onura46 I'm adding you as a "reviewer" for Backrooms which means it'll ping you if someone has an open (non-draft) PR that touches the Backroom mod files because you're its maintainer, if you don't want that I'll remove it tho

I'll have a look at this as soon as I can. Probably tonight. Thanks for the ping and the PR ❤️

[onura46]

Apologies for the delay. Works great for me! Really elegant way of cleaning up some of the Backrooms' idiosyncratic code, and paving the way for similar things in the future.

Btw, do these additions to mapgen options need to be written up in documentation?

[Procyonae]

Btw, do these additions to mapgen options need to be written up in documentation?

The map extra color + flag thing? They only affect inheritance which you'd think would have decent documentation but all we currently have is the nigh useless https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/JSON_INHERITANCE.md. I'd be down to write some proper per type inheritance documentation at some point it just takes quite a bit of digging to see how everything works.
Edit: Actually that's an awful excuse I'ma start writing proper docs for it now (not in this PR)