Major destabilisation, phase 2. This time it's the backends' turn:

each backend now stores all its internal variables in a big struct,
and each backend function gets a pointer to this struct passed to
it. This still isn't the end of the work - lots of subsidiary things
still use globals, notably all the cipher and compressor modules and
the X11 forwarding authentication stuff. But ssh.c itself has now
been transformed, and that was the really painful bit, so from here
on it all ought to be a sequence of much smaller and simpler pieces
of work.

[originally from svn r2127]

terminal.h

@@ -157,6 +157,9 @@ struct terminal_tag {
     wchar_t *paste_buffer;
     int paste_len, paste_pos, paste_hold;
     long last_paste;
+
+    void (*resize_fn)(void *, int, int);
+    void *resize_ctx;
 };
 
 #define in_utf(term) ((term)->utf || line_codepage==CP_UTF8)