Windows: reinstate redraws during interactive scrollbar drag.

I just discovered that they weren't happening, and the reason why is
thoroughly annoying. Details are in the long comment I've added to the
WM_VSCROLL handler in WndProc, but the short version is that when you
interactively drag the terminal window's scrollbar, a subsidiary
message loop is launched by DefWndProc, causing all our timer events
to go missing until the user lets go of the scrollbar again. So we
have to manually update the terminal window on scroll events, because
the normal system is out of action.

I assume this changed behaviour round about the big rework of terminal
updating in February. Good job I spotted it just _before_ 0.75, and
not just after!