Adjust text spacing for self ref arcs

[sverweij]

What this changes

• Uses 2px less line spacing then regular arcs. Regular arcs need this to step around the lines. For self reference arcs this won't happen since the whole label is rendered above the arc anyway.

  How this looks

  Sample

  # Alice & Bob classics: public key encryption
  msc {
    hscale="1.4", wordwraparcs=on;

    Alice, Bob;

    --- [label="public key authentication"];
    Alice => Alice [label="sign message with own private key"];
    Alice => Bob [label="signed message"];
    Bob   => Alice [label="get Alice her public key"];
    Alice >> Bob [label="Alices' public key"];
    Bob   => Bob [label="verify message with Alices' public key"];

    --- [label="public key encryption"];
    Alice => Bob [label="get Bobs' public key"];
    Bob   >> Alice [label="Bobs' public key"];
    Alice => Alice [label="encrypt message with Bob's public key"];
    Alice => Bob [label="encrypted message"];
    Bob   => Bob [label="decrypt message with own private key"];
  }

Before

After