Get screen dimensions directly from screen driver - without use of
a channel!

        SCRXY sizex%, sizey%

Where sizex%/sizey% are return parameters

Only programmer parameter errors returned.

Note: This has only been tested on SMSQ/E. I dont know if it is valid for

The source code is included with the toolkit. However, the library
file must be downloaded separately.

V0.01, pjw, 26 Oct 2018