Set the origin of the virtual display
void SLsmg_set_screen_start (int *r, int *c)
SLsmg_set_screen_start sets the origin of the virtual display
to the row *r and the column *c. If either r or c
is NULL, then the corresponding value will be set to 0.
Otherwise, the location specified by the pointers will be updated to
reflect the old origin.
See slang/demo/pager.c for how this function may be used to
scroll horizontally.
SLsmg_init_smg