GTK: fix crash changing font size when terminal maximised.

When I maximised a terminal window today and then used Ctrl-< to
reduce its font size (expecting that the window size would stay the
same but more characters would be squeezed in), pterm failed the
assertion in term_request_resize_completed() that checks
term->win_resize_pending == WIN_RESIZE_AWAIT_REPLY.

This happened because in this situation request_resize_internal() was
called from within window.c rather than from within the terminal code
itself. So the terminal didn't know a resize is pending at all, and
was surprised to be told that one had finished.

request_resize_internal() already has a flag parameter to tell it
whether a given resize came from the terminal or not. On the main code
path, that flag is used to decide whether to notify the terminal. But
on the early exit path when the window is maximised, we weren't
checking the flag. An easy fix.

(cherry picked from commit 95b926865a)

unix/window.c

@@ -2602,7 +2602,8 @@ static void request_resize_internal(GtkFrontend *inst, bool from_terminal,
 #endif
                      0)) {
             queue_toplevel_callback(gtkwin_deny_term_resize, inst);
-            term_resize_request_completed(inst->term);
+            if (from_terminal)
+                term_resize_request_completed(inst->term);
             return;
         }
     }