Fix some headless window sizes on Windows, to make `make check` more reliable

[Stephan Bergmann]

I wonder if anybody has any thoughts on 
<https://gerrit.libreoffice.org/c/core/+/140588> "Fix some headless 
window sizes on Windows, to make `make check` more reliable".  It's a 
bit of a hack, but seems to get its job done.  (And I'm not aware of any 
work to make something like the "true headless" Linux svp backend also 
available on macOS and Windows, which would make this hack moot?)