Fixed bug 1751 - Direcftb renderer viewport behaviour differs from API description.

tomaszewski.p

According to API description:
SDL_RenderSetViewport - Use this function to set the drawing area for rendering on the current target.

DirectFB renderer in DirectFB_UpdateViewport function just sets clip on surface, instead of moving drawing area.

Attached patch uses set clip rectangle to move {x,y} coordinates during draw and blit operations to be compatible with API description.