broadway: Add a setter for display scale

This is useful when using Broadway as a headless
display server.

Fixes: #3934

gdk/broadway/gdkbroadwaydisplay.h

@@ -48,6 +48,10 @@ void                    gdk_broadway_display_show_keyboard       (GdkBroadwayDis
 GDK_AVAILABLE_IN_ALL
 void                    gdk_broadway_display_hide_keyboard       (GdkBroadwayDisplay *display);
 
+GDK_AVAILABLE_IN_4_4
+void                    gdk_broadway_display_set_surface_scale   (GdkDisplay *display,
+                                                                  int         scale);
+
 G_END_DECLS
 
 #endif /* __GDK_BROADWAY_DISPLAY_H__ */

gdk/broadway/gdkdisplay-broadway.c

@@ -94,10 +94,12 @@ _gdk_broadway_display_size_changed (GdkDisplay                      *display,
 
   if (msg->width == current_size.width &&
       msg->height == current_size.height &&
-      msg->scale == broadway_display->scale_factor)
+      (msg->scale == broadway_display->scale_factor ||
+       broadway_display->fixed_scale))
     return;
 
-  broadway_display->scale_factor = msg->scale;
+  if (!broadway_display->fixed_scale)
+    broadway_display->scale_factor = msg->scale;
 
   gdk_monitor_set_geometry (monitor, &(GdkRectangle) { 0, 0, msg->width, msg->height });
   gdk_monitor_set_scale_factor (monitor, msg->scale);
@@ -328,6 +330,37 @@ gdk_broadway_display_hide_keyboard (GdkBroadwayDisplay *display)
   _gdk_broadway_server_set_show_keyboard (display->server, FALSE);
 }
 
+/**
+ * gdk_broadway_display_set_surface_scale:
+ * @display: (type GdkBroadwayDisplay): the display
+ * @scale: The new scale value, as an integer >= 1
+ *
+ * Forces a specific window scale for all windows on this display,
+ * instead of using the default or user configured scale. This
+ * is can be used to disable scaling support by setting @scale to
+ * 1, or to programmatically set the window scale.
+ *
+ * Once the scale is set by this call it will not change in
+ * response to later user configuration changes.
+ *
+ * Since: 4.4
+ */
+void
+gdk_broadway_display_set_surface_scale (GdkDisplay *display,
+                                        int         scale)
+{
+  GdkBroadwayDisplay *self;
+
+  g_return_if_fail (GDK_IS_BROADWAY_DISPLAY (display));
+  g_return_if_fail (scale > 0);
+
+  self = GDK_BROADWAY_DISPLAY (display);
+
+  self->scale_factor = scale;
+  self->fixed_scale = TRUE;
+  gdk_monitor_set_scale_factor (self->monitor, scale);
+}
+
 static GListModel *
 gdk_broadway_display_get_monitors (GdkDisplay *display)
 {

gdk/broadway/gdkdisplay-broadway.h

@@ -57,6 +57,7 @@ struct _GdkBroadwayDisplay
   GListStore *monitors;
   GdkMonitor *monitor;
   int scale_factor;
+  gboolean fixed_scale;
 
   GHashTable *texture_cache;