New Dark Hive transition rules for 1.16

Long story short, after octalot's proposed fix I decided to have a
closer loook at the Mycelium/Deprecated Mushroom Grove terrain graphics
rules in Wesnoth master right now, and it turns out that the Mycelium
base graphics are drawn in a Surprising™ fashion:

  {NEW:BASE Tb,*^Uf* forest/mushroom-base LAYER=-319}

Which means that in order to actually get our transitions into
Mycelium/MG, we need to draw on a higher layer than Mycelium's. So in
1.16 we need two different rules for each Dark Hive variation: one to
avoid drawing transitions in the wrong layer (which will set the
transition flag early) and another to force a different layer for those
terrains specifically.

How this will play out when both rules come into effect within the same
vicinity is unbeknownst to me at this time. I don't think there are any
situations where we'll see Mycelium neighbouring one of the terrains
blacklisted by the base rule, however, at least not in IftU and AtS.

Closes #11.

CC: project-ethea/Invasion_from_the_Unknown#73
CC: project-ethea/After_the_Storm#83

changelog.md

@@ -12,6 +12,12 @@ Version 20200816+dev:
   * Made `AMLA_BUG_4419_WORKAROUND` a no-op on 1.15.4 and later since mainline
     #4419 is solved there.
 
+* Terrains:
+  * Implemented a new Dark Hive transition logic to avoid issues with the
+    deprecated Mushroom Grove `^Uf*` terrain as well as the new Mycelium `T*`
+    base terrain proper when running add-ons on Wesnoth 1.15.2 and later
+    (issue #11, see also IftU #73 and AtS #83).
+
 
 Version 20200816:
 -----------------

terrain-graphics/_final.cfg

@@ -76,10 +76,39 @@
 
 # Dark hive transitions
 {NEW:BEACH                  (Yhr,Yhl,Yhs)               W*         flat/shore}
-# Don't replace certain defined transitions for id est road, sand, mountains...
-{TRANSITION_RESTRICTED_L        Yhr (!,Yhr,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds) -499 (dark-hive/floor)  }
-{TRANSITION_RESTRICTED_L        Yhl (!,Yhl,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds) -499 (dark-hive/lair)   }
-{TRANSITION_RESTRICTED_L        Yhs (!,Yhs,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds) -499 (dark-hive/surface)}
+
+# Don't replace transitions from specific "dominant" terrains such as rockbound
+# cave, sand, road, mountains, hills, etc.
+#
+# NOTE: (Naia #11, IftU #73, AtS #83)
+#
+# The deprecated Mushroom Grove gains compulsory Mycelium base graphics in
+# 1.16 with weird layering in order to enforce the Mycelium + Mushroom Grove
+# combination on almost all base terrains for no discernible reason. Wesnoth
+# devs, everyone!
+
+#ifver WESNOTH_VERSION < 1.15.2
+# No special rules for Mushroom Grove in 1.14 since it's not a stubborn
+# broken-by-design overlay in that version.
+{TRANSITION_RESTRICTED_L        Yhr (!,Yhr,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds)          -499 (dark-hive/floor)  }
+{TRANSITION_RESTRICTED_L        Yhl (!,Yhl,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds)          -499 (dark-hive/lair)   }
+{TRANSITION_RESTRICTED_L        Yhs (!,Yhs,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds)          -499 (dark-hive/surface)}
+#else
+# This makes it first so no transitions are drawn into Mycelium/Deprecated
+# Mushroom Grove.
+{TRANSITION_RESTRICTED_L        Yhr (!,Yhr,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds,T*,*^Uf*) -499 (dark-hive/floor)  }
+{TRANSITION_RESTRICTED_L        Yhl (!,Yhl,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds,T*,*^Uf*) -499 (dark-hive/lair)   }
+{TRANSITION_RESTRICTED_L        Yhs (!,Yhs,Uh,H*,M*,Q*,Xo*,Urb,Rr,Rp,Ds,T*,*^Uf*) -499 (dark-hive/surface)}
+# Now we make a special case for Mycelium/Deprecated Mushroom Grove (which is
+# drawn at layer -319 as of 1.15.4) and force our transitions over it.
+{TRANSITION_RESTRICTED_L        Yhr (T*,*^Uf*)                                     -318 (dark-hive/floor)  }
+{TRANSITION_RESTRICTED_L        Yhl (T*,*^Uf*)                                     -318 (dark-hive/lair)   }
+{TRANSITION_RESTRICTED_L        Yhs (T*,*^Uf*)                                     -318 (dark-hive/surface)}
+# Note that we don't need to interfere with the new Mushroom Grove overlays
+# (^T*) because they don't force base graphics and transitions down our
+# throats.
+#endif
+
 # HACK: Wesnoth 1.6 RC 1. Define a custom Re transition, as
 # the default mainline one is drawn below Yhs and looks awful.
 {TRANSITION_RESTRICTED_LF       Yhr (Re,Rb,Rd) -331    transition3           (dark-hive/floor)  }