Precision scrolling with mouse whell on MS-Windows

[Eli Zaretskii]

I've implemented this on MS-Windows.  Unfortunately, I cannot fully
test this, because I have no access to any system with such a mouse.
So someone else will have to test this and find all the bugs in my
code.

One note: unlike on other systems (I think), the MS-Windows
implementation is sensitive to user's system-wide setting of what is
"scroll unit".  This is normally a small number of screen lines (by
default, 3), but can also be "the whole page".  The code I wrote
computes the pixel deltas in as a fraction of this "scroll unit".

HTH