Dark Hive/Deprecated Mushroom Grove transition issues in 1.16

[irydacea]

There are some issues with Dark Hive transitions when the Deprecated Mushroom Grove (^Uf/^Ufi) is involved in the Wesnoth dev branch. Either the mushroom "base" transitions into Dark Hive when the mushroom overlays Dark Hive, or Dark Hive doesn't transition into the mushroom "base" when it doesn't.

AtS E1S9.3 NW (master@311b4a016e291211651bea3337531539fc9c7d9d):

AtS E1S9.3 NW (1.14@a5315aad658636610ff586b506a62ae07e45c975):

Note: Haven't tested the new Mushroom Grove yet. It's not in my best interest to deal with the technical and artistic complications that will arise from that until I no longer need to support Wesnoh 1.14 — see project-ethea/Invasion_from_the_Unknown#73 and project-ethea/After_the_Storm#83.

(CC #4)

[stevecotton]

Adding *^Uf* to terrain-graphics/_final.cfg#L79-L82 's lists of terrains that (IIUC) Dark Hive doesn't override the transitions for has the following results on 1.14 and 1.15; it seems to solve the issue without needing a version-specific fix.

There's still an issue for the Yhr^Uf hex, but I think that looks weird in the original version too; I read that graphic as either mushrooms growing through metal with no corrosion on the surrounding metal, or as mushrooms standing on a metal floor with no roots to support or feed them.