ipw: Fix race condition with polling the cancel state of the dialog

Sometimes the interthread process window dialog reports "cancelled"
when it was not actually cancelled, it was just hidden and reshown,
Since we are polling the cancel state interthread, we must reset
the cancel status BEFORE the dialog is shown, and never toggle the
cancel state unless it is a true cancel action by the user.

gtk2_ardour/interthread_progress_window.cc

@@ -35,6 +35,8 @@ InterthreadProgressWindow::InterthreadProgressWindow (ARDOUR::InterThreadInfo* i
 	: ArdourDialog (t, true)
 	, _interthread_info (i)
 {
+	_interthread_info->cancel = false;
+
 	_bar.set_orientation (Gtk::PROGRESS_LEFT_TO_RIGHT);
 
 	set_border_width (12);
@@ -50,8 +52,6 @@ InterthreadProgressWindow::InterthreadProgressWindow (ARDOUR::InterThreadInfo* i
 
 	set_default_size (200, 100);
 	show_all ();
-	hide ();
-	_interthread_info->cancel = false; // override on_hide
 
 	Glib::signal_timeout().connect (sigc::mem_fun (*this, &InterthreadProgressWindow::update), 100);
 }