Codefix: Clear m6 more thoroughly when making tiles.

m6 bits 6-7 were previously used to be for bridge above state, but should now be cleared.

src/object_map.h

@@ -80,7 +80,7 @@ inline void MakeObject(Tile t, Owner o, ObjectID index, WaterClass wc, uint8_t r
 	t.m3() = random;
 	t.m4() = 0;
 	t.m5() = index.base() >> 16;
-	SB(t.m6(), 2, 4, 0);
+	SB(t.m6(), 2, 6, 0);
 	t.m7() = 0;
 }