If the user holds down Alt-> so that the key repeats, then a second
call to change_font_size can occur while the window resize from the
previous one has yet to complete. This leads to the new pixel size of
the window from resize #1 being interpreted in the light of the font
size from reesize #2, so that the two get out of step and the
_character_ size of the terminal changes as a result.
The simplest fix is to disallow starting a second font-size-based
window resize while the first is still in flight - which, now that the
'win_resize_pending' flag lives in window.c and not terminal.c, is
easy to achieve.
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