Keep window size in window position check

[KSR-Yasuda]

On switching current window in window-focus mode,
it checks movable direction by trying wincmd {h,j,k,l}.

It switches current window, so, if the window size is smaller than &winwidth and &winheight,
the window is resized.

The setting &winwidth is 20 by default, so vertically small window is likely to be resized.

Procedure to Repro

1. :set winwidth=20
2. :below vsplit
3. :wincmd | (Change the other window size smaller than &winwidth)
4. Launch winresizer (Ctrl-q)
5. Switch to window-focus mode (f)