More review changes and added lackings

-Removed unnecessary special handling of terrain-inducing overworld weather states from ChangeTypeBasedOnTerrain.
-Mimicry now activates normally for terrain-inducing overworld weather states.
-Weather abilities now activate normally when switchineffects is called, and at the beginning of a battle.
-Terrain abilities now activate normally at the beginning of a battle.

data/battle_scripts_1.s

@@ -8629,6 +8629,7 @@ BattleScript_TerrainSeedLoop_NextBattler:
 	addbyte gBattlerTarget, 0x1
 	jumpifbytenotequal gBattlerTarget, gBattlersCount, BattleScript_TerrainSeedLoopIter
 	restoretarget
+	call BattleScript_ActivateSwitchInAbilities
 	return
 
 BattleScript_ActivateSwitchInAbilities:

src/battle_main.c

@@ -3720,6 +3720,10 @@ static void TryDoEventsBeforeFirstTurn(void)
         gBattlerAttacker = gBattlerByTurnOrder[gBattleStruct->switchInAbilitiesCounter++];
         if (AbilityBattleEffects(ABILITYEFFECT_ON_SWITCHIN, gBattlerAttacker, 0, 0, 0) != 0)
             return;
+        if (gBattleWeather & B_WEATHER_ANY && WEATHER_HAS_EFFECT && AbilityBattleEffects(ABILITYEFFECT_ON_WEATHER, gActiveBattler, 0, 0, 0) != 0)
+            return;
+        if (gFieldStatuses & STATUS_FIELD_TERRAIN_ANY && AbilityBattleEffects(ABILITYEFFECT_ON_TERRAIN, gActiveBattler, 0, 0, 0) != 0)
+            return;
     }
     if (AbilityBattleEffects(ABILITYEFFECT_TRACE1, 0, 0, 0, 0) != 0)
         return;

src/battle_script_commands.c

@@ -6785,6 +6785,7 @@ static void Cmd_switchineffects(void)
         gDisableStructs[gActiveBattler].truantSwitchInHack = 0;
 
         if (AbilityBattleEffects(ABILITYEFFECT_ON_SWITCHIN, gActiveBattler, 0, 0, 0)
+         || (gBattleWeather & B_WEATHER_ANY && WEATHER_HAS_EFFECT && AbilityBattleEffects(ABILITYEFFECT_ON_WEATHER, gActiveBattler, 0, 0, 0))
          || (gFieldStatuses & STATUS_FIELD_TERRAIN_ANY && AbilityBattleEffects(ABILITYEFFECT_ON_TERRAIN, gActiveBattler, 0, 0, 0))
          || ItemBattleEffects(ITEMEFFECT_ON_SWITCH_IN, gActiveBattler, FALSE)
          || AbilityBattleEffects(ABILITYEFFECT_TRACE2, 0, 0, 0, 0)

src/battle_util.c

@@ -4298,10 +4298,6 @@ static u8 ForewarnChooseMove(u32 battler)
 bool8 ChangeTypeBasedOnTerrain(u8 battlerId)
 {
     u8 battlerType;
-    u16 terrainFlags = VarGet(VAR_TERRAIN) & STATUS_FIELD_TERRAIN_ANY;
-
-    if (terrainFlags && gFieldStatuses & STATUS_FIELD_TERRAIN_ANY)
-        gFieldStatuses = terrainFlags | STATUS_FIELD_TERRAIN_PERMANENT; // terrain is permanent
 
     if (gFieldStatuses & STATUS_FIELD_ELECTRIC_TERRAIN)
         battlerType = TYPE_ELECTRIC;