Set splash screen to non-resizable so it displays correctly in smart window managers.

There is an attempt in the code to handle the situation of being allocated more space, but it does not work correctly.  The splash isn't resizable anyway, this fix makes it appear as it should.

git-svn-id: svn://localhost/ardour2/branches/3.0@13111 d708f5d6-7413-0410-9779-e7cbd77b26cf

gtk2_ardour/splash.cc

@@ -78,6 +78,7 @@ Splash::Splash ()
 	add (darea);
 
 	set_default_size (pixbuf->get_width(), pixbuf->get_height());
+	set_resizable (false);
 	the_splash = this;
 
         expose_done = false;