Add dash-dot-dot lines for HPGL.

Also converts HPGL line types to fixed (rather than adaptive).  Varying
the pattern to fit each segment is going to look bad most of the time,
but particularly when stroking arcs or circles where HPGL will try to
repeat it for each chord segment.

common/plotters/HPGL_plotter.cpp

@@ -923,18 +923,11 @@ const char* HPGL_PLOTTER::lineTypeCommand( PLOT_DASH_TYPE linetype )
 {
     switch( linetype )
     {
-    case PLOT_DASH_TYPE::DASH:
-        return "LT -2 4 1;";
-        break;
-    case PLOT_DASH_TYPE::DOT:
-        return "LT -1 2 1;";
-        break;
-    case PLOT_DASH_TYPE::DASHDOT:
-        return "LT -4 6 1;";
-        break;
-    default:
-        return "LT;";
-        break;
+    case PLOT_DASH_TYPE::DASH:       return "LT 2 4 1;";
+    case PLOT_DASH_TYPE::DOT:        return "LT 1 1 1;";
+    case PLOT_DASH_TYPE::DASHDOT:    return "LT 4 6 1;";
+    case PLOT_DASH_TYPE::DASHDOTDOT: return "LT 7 8 1;";
+    default:                         return "LT;";
     }
 }