Codechange: Add fast path to catenary drawing in MaskWireBits

MaskWireBits always returns its input unchanged if the input
has only 0 or 1 track bits set.
Having only 0 or 1 track bits sets (i.e. non junction tiles)
is by far the most common case.
Examining the state of neighbouring tiles and the subsequent
masking logic is relatively expensive and can be omitted in this case.

src/elrail.cpp

@@ -124,6 +124,9 @@ static TrackBits GetRailTrackBitsUniversal(TileIndex t, byte *override)
  */
 static TrackBits MaskWireBits(TileIndex t, TrackBits tracks)
 {
+	/* Single track bits are never masked out. */
+	if (likely(HasAtMostOneBit(tracks))) return tracks;
+
 	if (!IsPlainRailTile(t)) return tracks;
 
 	TrackdirBits neighbour_tdb = TRACKDIR_BIT_NONE;