Fix maximum size of .ico files in ICOReader::write()

According to MSDN, a maximum icon size is 256 pixels.

http://blogs.msdn.com/b/oldnewthing/archive/2010/10/18/10077133.aspx



Change-Id: Id87c89a20c9cba6ef041b49f93f84c5e9c3eb79f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>