C23 compatibility

In C23, empty paramter list in function declaration actually means that
function takes parameters. Update to declare parameters for all
functions that take them, and pass NULL as needed.

Without this change, building with C23 fails with errors like shown
below. These broke build for Fedora 42, which comes with GCC 15 and
uses C23 by default.

    vikaggregatelayer.c: In function ‘vik_aggregate_layer_create’:
    vikaggregatelayer.c:631:27: error: too many arguments to function ‘vik_aggregate_layer_new’; expected 0, have 1
      631 |   VikAggregateLayer *rv = vik_aggregate_layer_new (vp);
          |                           ^~~~~~~~~~~~~~~~~~~~~~~  ~~
    In file included from viking.h:57:
    vikaggregatelayer.h:48:20: note: declared here
       48 | VikAggregateLayer *vik_aggregate_layer_new ();
          |                    ^~~~~~~~~~~~~~~~~~~~~~~
    vikaggregatelayer.c: In function ‘aggregate_layer_unmarshall’:
    vikaggregatelayer.c:680:27: error: too many arguments to function ‘vik_aggregate_layer_new’; expected 0, have 1
      680 |   VikAggregateLayer *rv = vik_aggregate_layer_new(vvp);
          |                           ^~~~~~~~~~~~~~~~~~~~~~~ ~~~
    vikaggregatelayer.h:48:20: note: declared here
       48 | VikAggregateLayer *vik_aggregate_layer_new ();
          |                    ^~~~~~~~~~~~~~~~~~~~~~~
    vikaggregatelayer.c: At top level:
    vikaggregatelayer.c:700:20: error: conflicting types for ‘vik_aggregate_layer_new’; have ‘VikAggregateLayer *(VikViewport *)’ {aka ‘struct _VikAggregateLayer *(struct _VikViewport *)’}
      700 | VikAggregateLayer *vik_aggregate_layer_new (VikViewport *vvp)
          |                    ^~~~~~~~~~~~~~~~~~~~~~~
    vikaggregatelayer.h:48:20: note: previous declaration of ‘vik_aggregate_layer_new’ with type ‘VikAggregateLayer *(void)’ {aka ‘struct _VikAggregateLayer *(void)’}
       48 | VikAggregateLayer *vik_aggregate_layer_new ();
          |                    ^~~~~~~~~~~~~~~~~~~~~~~

src/vikaggregatelayer.h

@@ -45,7 +45,7 @@ GType vik_aggregate_layer_get_type ();
 typedef struct _VikAggregateLayer VikAggregateLayer;
 
 void vik_aggregate_layer_uninit ();
-VikAggregateLayer *vik_aggregate_layer_new ();
+VikAggregateLayer *vik_aggregate_layer_new ( VikViewport *vvp );
 void vik_aggregate_layer_add_layer ( VikAggregateLayer *val, VikLayer *l, gboolean allow_reordering );
 void vik_aggregate_layer_insert_layer ( VikAggregateLayer *val, VikLayer *l, VikLayer *crt_layer, gboolean allow_reordering );
 void vik_aggregate_layer_move_layer ( VikAggregateLayer *val, GtkTreeIter *child_iter, gboolean up );

src/viklayerspanel.c

@@ -659,7 +659,7 @@ static void vik_layers_panel_init ( VikLayersPanel *vlp )
   vlp->hbox = gtk_hbox_new ( TRUE, 2 );
   vlp->vt = vik_treeview_new ( );
 
-  vlp->toplayer = vik_aggregate_layer_new ();
+  vlp->toplayer = vik_aggregate_layer_new ( NULL );
   vik_layer_rename ( VIK_LAYER(vlp->toplayer), _("Top Layer"));
   g_signal_connect_swapped ( G_OBJECT(vlp->toplayer), "update", G_CALLBACK(layers_panel_emit_update), vlp );
 

test/test_file_load.c

@@ -51,7 +51,7 @@ int main(int argc, char *argv[])
   // Seems to work without an $DISPLAY
   // Also get lots of warnings about no actual drawing GCs available
   // but for file processing this seems to be good enough
-  VikAggregateLayer* agg = vik_aggregate_layer_new ();
+  VikAggregateLayer* agg = vik_aggregate_layer_new ( NULL );
   VikViewport* vp = vik_viewport_new ();
 
   VikLoadType_t lt = a_file_load ( agg, vp, NULL, argv[1], TRUE, external, NULL );

test/vik2vik.c

@@ -43,7 +43,7 @@ int main(int argc, char *argv[])
   // Also get lots of warnings about no actual drawing GCs available
   // but for file processing this seems to be good enough
   VikLoadType_t lt;
-  VikAggregateLayer* agg = vik_aggregate_layer_new ();
+  VikAggregateLayer* agg = vik_aggregate_layer_new ( NULL );
 #if GTK_CHECK_VERSION (3,0,0)
   VikWindow *vw = vik_window_new_window();
   VikViewport* vp = vik_window_viewport(vw);