x11: Store the mouse button serial for emulated pointer events as well

Otherwise, filtered emulated button events, such as for mouse wheels, can slip through the core event handler.

(cherry picked from commit f48525aa703e0e11134347b38571661d6ca829fd)

src/video/x11/SDL_x11xinput2.c

@@ -590,6 +590,9 @@ void X11_HandleXinput2Event(SDL_VideoDevice *_this, XGenericEventCookie *cookie)
         bool pointer_emulated = false;
 #endif
 
+        // Store the button serial to filter out redundant core button events.
+        videodata->xinput_last_button_serial = xev->serial;
+
         if (pen) {
             if (xev->deviceid != xev->sourceid) {
                 // Discard events from "Master" devices to avoid duplicates.
@@ -607,9 +610,6 @@ void X11_HandleXinput2Event(SDL_VideoDevice *_this, XGenericEventCookie *cookie)
             SDL_WindowData *windowdata = X11_FindWindow(videodata, xev->event);
             int x_ticks = 0, y_ticks = 0;
 
-            // Store the button serial to filter out redundant core button events.
-            videodata->xinput_last_button_serial = xev->serial;
-
             if (xev->deviceid != videodata->xinput_master_pointer_device) {
                 // Ignore slave button events on non-focused windows, or focus can be incorrectly set while a grab is active.
                 if (SDL_GetMouseFocus() != windowdata->window) {