Previously, because the video (client area) was centered but the top
and bottom borders are uneven (title is taller), then if the window
is shrunk vertically to just-fit the desktop - the top edge of the
title bar ended above the top edge of the display.
This is a state which Windows prevents during manual move, but
apparently it's not rejected at the Windows API.
Now we ensure it doesn't happen, and nudge the window down to align
the top edges if necessary.
This is a commulative regression of commits 981048e0 and 364af7c6.
To clarify functionality, this includes a no-op change: fit_rect was
renamed to fit_rect_size and it now takes explicit width and height,
because it only used the width/height of rc2 anyway.
Fixes#6695
e00ae12bbb