convert: minor optimisation to conversion from VYU
convert: minor optimisation to conversion from VYU

Signed-off-by: André Draszik <4a4cb09e7b43b167f95b1dc166b9a7dec9f65d7c@st.com>

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