Added static function sf::GlResource::releaseThreadResources();

If you use a thread with rendering contexts: e.g. repeatedly activating and disabling a window for a thread, and then deleting that thread, will not cause the global context to flush its invalid elements.

I have added a simple method for doing just that. Of course, this should be implemented using RAII in some way, perhaps through thread local objects?

A post on the SFML forum can be found here: http://en.sfml-dev.org/forums/index.php?topic=10309.msg77821#msg77821