As a macro, it always refers to the global grd_curcanv. This interferes
with converting canvas handling to be an argument. Expand GHEIGHT so
that uses of grd_curcanv can be changed individually.
As a macro, it always refers to the global grd_curcanv. This interferes
with converting canvas handling to be an argument. Expand GWIDTH so
that uses of grd_curcanv can be changed individually.