Shift the reference scale to be generated from the centre of the disp…

…lay.

Previously this was done at the latitude of the bottom of the display.
For most zoomed in usage it makes no practical difference,
 but when zoomed out it's more logical as it's on average nearer to actual scale at locations across the visible map.

src/vikviewport.c

@@ -492,8 +492,8 @@ void vik_viewport_draw_scale ( VikViewport *vvp )
     PangoLayout *pl;
     gchar s[128];
 
-    vik_viewport_screen_to_coord ( vvp, 0, vvp->height, &left );
-    vik_viewport_screen_to_coord ( vvp, vvp->width/SCSIZE, vvp->height, &right );
+    vik_viewport_screen_to_coord ( vvp, 0, vvp->height/2, &left );
+    vik_viewport_screen_to_coord ( vvp, vvp->width/SCSIZE, vvp->height/2, &right );
 
     vik_units_distance_t dist_units = a_vik_get_units_distance ();
     switch (dist_units) {