* Draws a tunnel of bridge tile.

  * For tunnels, this is rather simple, as you only needa draw the entrance.

  * Bridges are a bit more complex. base_offset is where the sprite selection comes into play

  * and it works a bit like a bitmask.<p> For bridge heads:

  * <ul><li>Bit 0: direction</li>

  * <li>Bit 1: northern or southern heads</li>

  * <li>Bit 2: Set if the bridge head is sloped</li>

  * <li>Bit 3 and more: Railtype Specific subset</li>

  * </ul>

  * For middle parts:

  * <ul><li>Bits 0-1: need to be 0</li>

  * <li>Bit 2: direction</li>

  * <li>Bit 3 and above: Railtype Specific subset</li>

  * </ul>

  * Please note that in this code, "roads" are treated as railtype 1, whilst the real railtypes are 0, 2 and 3

  */