x11: Dynamically update the scale factor

If the text-scaling-factor setting is available via D-Bus, add a listener and update the content scale values for the displays if the value is changed during runtime.

Factors out the D-Bus message pump from the system theme detection code to the general D-Bus code, as it's now used for more purposes than just the system theme.

src/core/linux/SDL_dbus.c

@@ -498,4 +498,16 @@ SDL_DBus_ScreensaverInhibit(SDL_bool inhibit)
 
     return SDL_TRUE;
 }
+
+void SDL_DBus_PumpEvents(void)
+{
+    if (dbus.session_conn) {
+        dbus.connection_read_write(dbus.session_conn, 0);
+
+        while (dbus.connection_dispatch(dbus.session_conn) == DBUS_DISPATCH_DATA_REMAINS) {
+            /* Do nothing, actual work happens in DBus_MessageFilter */
+            SDL_DelayNS(SDL_US_TO_NS(10));
+        }
+    }
+}
 #endif

src/core/linux/SDL_dbus.h

@@ -94,6 +94,8 @@ extern SDL_bool SDL_DBus_QueryPropertyOnConnection(DBusConnection *conn, const c
 extern void SDL_DBus_ScreensaverTickle(void);
 extern SDL_bool SDL_DBus_ScreensaverInhibit(SDL_bool inhibit);
 
+extern void SDL_DBus_PumpEvents(void);
+
 #endif /* HAVE_DBUS_DBUS_H */
 
 #endif /* SDL_dbus_h_ */

src/core/linux/SDL_system_theme.c

@@ -158,18 +158,3 @@ SDL_SystemTheme_Get(void)
 {
     return system_theme_data.theme;
 }
-
-void
-SDL_SystemTheme_PumpEvents(void)
-{
-    SDL_DBusContext *dbus = system_theme_data.dbus;
-    DBusConnection *conn;
-    if (dbus == NULL) return;
-    conn = dbus->session_conn;
-    dbus->connection_read_write(conn, 0);
-
-    while (dbus->connection_dispatch(conn) == DBUS_DISPATCH_DATA_REMAINS) {
-        /* Do nothing, actual work happens in DBus_MessageFilter */
-        usleep(10);
-    }
-}

src/core/linux/SDL_system_theme.h

@@ -26,6 +26,5 @@
 
 extern SDL_bool SDL_SystemTheme_Init(void);
 extern SDL_SystemTheme SDL_SystemTheme_Get(void);
-extern void SDL_SystemTheme_PumpEvents(void);
 
 #endif /* SDL_system_theme_h_ */