IDirectFBSurface: Fix WaitForBackBuffer() for single buffer case.
IDirectFBSurface: Fix WaitForBackBuffer() for single buffer case.

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