This prevents stale drawing from being revealed if the window later grows.

For instance, 19 ppem "VL Gothic" on Windows, the advance value is 10
for half-width characters, but 18 for full-width characters.  In this
case, tmAveCharWidth is 9. Thus using tmAveCharWidth is reasonable.

This lets programs ask the user to save documents, etc., and possibly cancel
the quit.