patch 9.0.1927: patch 1916 (fixed terminal size) not optimal

Problem:  patch 1916 (fixed terminal size) not optimal
Solution: Add defines to make it easier changeable later

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/terminal.c b/src/terminal.c
index 991f056..7156665 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -272,10 +272,10 @@
     }
     *rows = atoi((char *)wp->w_p_tws);
     *cols = atoi((char *)p + 1);
-    if (*rows > 1000)
-	*rows = 1000;
-    if (*cols > 1000)
-	*cols = 1000;
+    if (*rows > VTERM_MAX_ROWS)
+	*rows = VTERM_MAX_ROWS;
+    if (*cols > VTERM_MAX_COLS)
+	*cols = VTERM_MAX_COLS;
     return minsize;
 }