vo_direct3d: always clear window before drawing video

This fixes the issue that black borders (e.g. on fullscreen) are not
redrawn, even if OSD rendering changes these areas.

In theory, the code could try some clever things to determine whether
clearing the window is really necessary, but that's probably not worth
the trouble and won't bring any significant performance gain, or might
even make things slower (because the GPU can't discard the old
contents).

Also fix redrawing when changing panscan with OSD disabled.