new release of Screen

I have attached to this message a new version of Screen.dll which includes all
previous updates and new fixes for the problems listed below. One side effect of
using this new version of Screen is that WhiteIndex.m and BlackIndex.m should now
work when passed screen numbers.

- Screen(..'Rect'..) when passed a pointer to an offscreen window would return
nonsense.
- FillRect and other drawing commands issued the warning "could not select
Palette for use" if the window in which you were drawing was an offscreen window
which had been opened using a screen number instead of a window pointer.
- Screen(..'PixelSize'..) now correctly returns the pixel depth for all windows.

--Allen