Applied a great patch from Maurizio Monge <monge@sns.it>.
Applied a great patch from Maurizio Monge <monge@sns.it>.

When repainting the stack due to a window update, e.g. a Flip(),
skip those regions which are hidden by opaque windows above the window
that changed.

This increases performance and reduces artifacts.

Example: Terminal with tons of debug messages under opaque window caused
the opaque window to be updated, too, within the intersection of both windows.

Another example: Run a fullscreen app in windowed mode, e.g. df_andi, and
see how frame rate increases as more of df_andi is hidden by opaque windows ;)

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