Rewrite stairs code.

[fozolo]

• Handle stairsplus stairs and MTG stairs in the same file.
• Only create stairs using stairsplus or MTG stairs, not both.
• Handle map switching from MTG stairs to stairsplus automatically.
• If using stairsplus:
  • Move stairsplus nodes to proper namespace.
  • Alias stairsplus nodes that moved to a new namespace.
  • Alias duplicated ice and snowblock stairs and slabs to their new namespace.
  • Make ice_brick sawable too.
  • Unify stairsplus cut ice and default:ice texture brightness.
• If using MTG stairs:
  • Move stairs and slabs to MTG stairs namespace.
  • Alias stairs and slabs that moved to the new namespace.

[HybridDog]

Thank you for revising this. MTG itself also registers ice and snowblock stairs: https://github.com/minetest/minetest_game/blob/master/mods/stairs/init.lua#L784 So when using MTG stairs, the original ones are overriden. Is this intended?

[fozolo]

83      if not minetest.registered_nodes["stairs:stair_" .. name] then
84          stairs.register_stair(name, nodename, groups, images, desc_stair, sounds)
85      end
86
87      if not minetest.registered_nodes["stairs:slab_" .. name] then
88          stairs.register_slab(name, nodename, groups, images, desc_slab, sounds)
89      end

Yes, they are intentionally overridden to adjust their groups and the use_texture_alpha property. The check on lines 83 and 87 to prevent duplicate registrations was in fact a bug left over from some other experiment. This prevented the new groups from being set for ice and snowblock and has now been corrected.

Moreblocks stairsplus may soon register ice and snowblock as well (minetest-mods/moreblocks#109), but they are intentionally overridden for the same reasons.