x11: Don't try to capture mouse input via X when XInput2 is in use

Trying to capture the pointer via XGrabPointer() when XInput2 is in use will always fail with 'AlreadyGrabbed', since the pointer is already grabbed by XInput2.

src/video/x11/SDL_x11mouse.c

@@ -391,13 +391,19 @@ static int X11_CaptureMouse(SDL_Window *window)
 
     if (window) {
         SDL_WindowData *data = window->driverdata;
-        const unsigned int mask = ButtonPressMask | ButtonReleaseMask | PointerMotionMask | FocusChangeMask;
-        Window confined = (data->mouse_grabbed ? data->xwindow : None);
-        const int rc = X11_XGrabPointer(display, data->xwindow, False,
-                                        mask, GrabModeAsync, GrabModeAsync,
-                                        confined, None, CurrentTime);
-        if (rc != GrabSuccess) {
-            return SDL_SetError("X server refused mouse capture");
+
+        /* If XInput2 is handling the pointer input, non-confinement grabs will always fail with 'AlreadyGrabbed',
+         * since the pointer is being grabbed by XInput2.
+         */
+        if (!data->xinput2_mouse_enabled || data->mouse_grabbed) {
+            const unsigned int mask = ButtonPressMask | ButtonReleaseMask | PointerMotionMask | FocusChangeMask;
+            Window confined = (data->mouse_grabbed ? data->xwindow : None);
+            const int rc = X11_XGrabPointer(display, data->xwindow, False,
+                                            mask, GrabModeAsync, GrabModeAsync,
+                                            confined, None, CurrentTime);
+            if (rc != GrabSuccess) {
+                return SDL_SetError("X server refused mouse capture");
+            }
         }
     } else if (mouse_focus) {
         SDL_UpdateWindowGrab(mouse_focus);