core: fix race condition between dfb_wm_close_all_stacks() and layer_context destructor
core: fix race condition between dfb_wm_close_all_stacks() and layer_context destructor

Distrotech/DirectFB,djbclark/directfb-core-DirectFB,DirectFB/directfb,lancebaiyouview/DirectFB,kaostao/directfb,jcdubois/DirectFB,lancebaiyouview/DirectFB,mtsekm/test,deniskropp/DirectFB,kevleyski/DirectFB-1,kaostao/directfb,kevleyski/directfb,deniskropp/DirectFB,sklnet/DirectFB,djbclark/directfb-core-DirectFB,kevleyski/DirectFB-1,DirectFB/directfb,kevleyski/directfb,deniskropp/DirectFB,jcdubois/DirectFB,djbclark/directfb-core-DirectFB,kevleyski/directfb,lancebaiyouview/DirectFB,mtsekm/test,sklnet/DirectFB,dfbdok/DirectFB1,dfbdok/DirectFB1,DirectFB/directfb,kevleyski/directfb,deniskropp/DirectFB,jcdubois/DirectFB,sklnet/DirectFB,mtsekm/test,Distrotech/DirectFB,lancebaiyouview/DirectFB,kevleyski/DirectFB-1,kevleyski/DirectFB-1,sklnet/DirectFB,kaostao/directfb,djbclark/directfb-core-DirectFB,Distrotech/DirectFB,dfbdok/DirectFB1