bug#987: x-display-pixel-width not returning correct value after resolution change

[Baoqiu Cui]

It seems that function x-display-pixel-width behaves a little
differently in GNU Emacs and XEmacs.

In XEmacs, (x-display-pixel-width) can recognize the display resolution
changes after XEmacs is started.  For example, if the display resolution
was originally set to 1400x1050 when XEmacs was started,
(x-display-pixel-width) would return 1400; then, if the display
resolution is changed to 1280x1024 while XEmacs is still running,
(x-display-pixel-width) would return 1280.

However, (x-display-pixel-width) on GNU Emacs is NOT able to recognize
such physical display resolution change (from 1400x1050 to 1280x1024)
and will still return 1400.

I am not sure if this is a bug or feature on GNU Emacs, but I would like
to see this function work like its counterpart in XEmacs.

Regards,
Baoqiu