Xlib: DisplayWidth / DisplayHeight

[Vladimir Dergachev]

Just to make it easier for anyone who is reading this thread in the 
archives:

At the moment the library you need is libxrandr (on Ubuntu install with
apt install libxrandr-dev), and read the man page Xrandr.

It is also useful to read the paper describing Xrandr protocol:

https://cgit.freedesktop.org/xorg/proto/randrproto/tree/randrproto.txt

best

Vladimir Dergachev