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

        SCRXY limX%, limY%

Where limX% / limY% are return parameters only

Parameter errors trigger the standard error reponse.


If the Pointer Interface (PI) is not present the "standard" values, ie
512 and 256, are returned instead. Clearly, this wont be correct if you
have a 768x512 screen size and no PI! This, however, appears to be how
TK2's SDUMP does things, so Im happy with that.

Software status:

V0.01, pjw, 26 Oct 2018
V0.02, pjw, June 5th 2019, check parameters, now Qdos + PI compatible
V0.03, pjw, 2021 Oct 03, if no PI return "standard" sizes

Conditions and DISCLAIMER as per