Contruct a new subwindow with
constraints of height lines, width columns, begin at
top line, and begin left most column.
static VALUE
window_subwin(VALUE obj, VALUE height, VALUE width, VALUE top, VALUE left)
{
struct windata *winp;
WINDOW *window;
VALUE win;
int h, w, t, l;
h = NUM2INT(height);
w = NUM2INT(width);
t = NUM2INT(top);
l = NUM2INT(left);
GetWINDOW(obj, winp);
window = subwin(winp->window, h, w, t, l);
win = prep_window(rb_obj_class(obj), window);
return win;
}