Previously, the Windows implementation of win_request_resize would
call term_size() to tell the terminal about its new size, _before_
calling SetWindowPos to actually change the window size.
If SetWindowPos ends up not getting the exact window size it asks for,
Windows notifies the application by sending back a WM_SIZE message -
synchronously, by calling back to the window procedure from within
SetWindowPos. So after the first over-optimistic term_size(), the
WM_SIZE handler would trigger a second one, resetting the terminal
again to the _actual_ size.
This was more or less harmless, since current handling of terminal
resizes is such that no terminal data gets too confused: any lines
pushed into the scrollback by the first term_size will be pulled back
out by the second one if needed (or vice versa), and since commit
271de3e4ec5682e, individual termlines are not eagerly truncated by
resizing them twice.
However, it's more work than we really wanted to be doing, and it
seems presumptuous to send term_size() before we know it's right! So
now we send term_size() after SetWindowPos returns, unless it already
got sent by a re-entrant call to the WM_SIZE handler _before_
SetWindowPos returned. That way, the terminal should get exactly one
term_size() call in response to win_request_resize(), whether it got
the size it was expecting or not.
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