Fix interpolating to identity bone overrides (#16609)

The previous code immediately dropped identity overrides, even if there still was an interpolation to be done.
Also a little bit of cleanup, and setting an appropriate identity default for the scale property when interpolating.

For modders: As a workaround, you can add a tiny offset so that overrides aren't identity overrides.

src/script/lua_api/l_object.cpp

@@ -668,7 +668,7 @@ int ObjectRef::l_set_bone_override(lua_State *L)
 
 		lua_getfield(L, -1, "interpolation");
 		if (lua_isnumber(L, -1))
-			prop.interp_timer = lua_tonumber(L, -1);
+			prop.interp_duration = lua_tonumber(L, -1);
 		lua_pop(L, 1);
 	};
 
@@ -718,7 +718,7 @@ static void push_bone_override(lua_State *L, const BoneOverride &props)
 		lua_newtable(L);
 		push_v3f(L, vec);
 		lua_setfield(L, -2, "vec");
-		lua_pushnumber(L, prop.interp_timer);
+		lua_pushnumber(L, prop.interp_duration);
 		lua_setfield(L, -2, "interpolation");
 		lua_pushboolean(L, prop.absolute);
 		lua_setfield(L, -2, "absolute");