bug#21173: 25.0.50; New frames positioned off screen with multiple monitors

[martin rudalics]

> This patch fixes the problem for me under Windows.  Many thanks to Andy
> Moreton!  Since Andy reports that negative X offsets also happen under
> the X Window system, should this also be fixed in function
> x_calc_absolute_position in src/term.c?

Thanks for testing, Francis.  AFAICT nobody reported a similar problem
on X so far.  So Andy please resend us your patch as attachment and
provide some suitable ChangeLog entry and I will install it (unless you
can already do that yourself).

Thanks, martin