inc: normalise capitalisation and arg renaming in inc/win/windowsx.bi to match original/fbfrog

The manual changes that had been applied to this file were in TitleCase,
except there were many instances of "cast(hwnd," instead of "cast(HWND,"

Also, a few parameters had been manually prefixed with _ but they are normally
automatically postfixed by fbfrog.

This normalisation minimises future diffs.

Author: rversteegen