patch 8.0.0898: can't use the alternate screen in a terminal window

Problem:    Can't use the alternate screen in a terminal window.
Solution:   Initialze the alternate screen. (Yasuhiro Matsumoto, closes
            #1957)  Add term_getaltscreen().

src/terminal.c

@@ -146,6 +146,8 @@ struct terminal_S {
 
     VTermPos	tl_cursor_pos;
     int		tl_cursor_visible;
+
+    int		tl_using_altscreen;
 };
 
 #define TMODE_ONCE 1	    /* CTRL-\ CTRL-N used */
@@ -1316,6 +1318,11 @@ handle_settermprop(
 	    out_flush();
 	    break;
 
+	case VTERM_PROP_ALTSCREEN:
+	    /* TODO: do anything else? */
+	    term->tl_using_altscreen = value->boolean;
+	    break;
+
 	default:
 	    break;
     }
@@ -1865,6 +1872,9 @@ create_vterm(term_T *term, int rows, int cols)
 
     /* Required to initialize most things. */
     vterm_screen_reset(screen, 1 /* hard */);
+
+    /* Allow using alternate screen. */
+    vterm_screen_enable_altscreen(screen, 1);
 }
 
 /*
@@ -1938,6 +1948,19 @@ term_get_buf(typval_T *argvars)
     return buf;
 }
 
+/*
+ * "term_getaltscreen(buf)" function
+ */
+    void
+f_term_getaltscreen(typval_T *argvars, typval_T *rettv)
+{
+    buf_T	*buf = term_get_buf(argvars);
+
+    if (buf == NULL)
+	return;
+    rettv->vval.v_number = buf->b_term->tl_using_altscreen;
+}
+
 /*
  * "term_getattr(attr, name)" function
  */