Fix AltGR getting stuck on Windows right alt-tab

Fixes #28511.

(cherry picked from commit e01452adb0ac51bbbb0304773b1efd28ca1a3f2d)

platform/windows/os_windows.cpp

@@ -356,6 +356,7 @@ LRESULT OS_Windows::WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
 				alt_mem = false;
 				control_mem = false;
 				shift_mem = false;
+				gr_mem = false;
 			} else { // WM_INACTIVE
 				input->release_pressed_events();
 				main_loop->notification(MainLoop::NOTIFICATION_WM_FOCUS_OUT);