Stipples and scroll optimizations

[Po Lu]

When a glyph with a stipple background is displayed in the scrolled area
of the screen, the optimization in `scrolling_window' should not be
performed.  This is because stipples are "tiled" across the screen from
a constant origin, so the display of the stipple might not be correct
after it is copied to a different part of the frame.

That problem can easily be seen by setting the stipple of the default
face to `dimple1' and scrolling a little.

WDYT about such a change?