Updated events test to work with multiple windows.

Press return to create a new window (up to 16 windows supported)
OSX: Fixed window destruction (apparently)

[NSWindow close] does not seem to release its contentview if that has been
set by hand so lets just keep the pointer around and release it ourselves.
OSX: expose callback now resembles X11

Moved calling of the expose callback to NSView drawRect to match X11.
Updated events test to load gl functions in the expose callback so
it does not crash when glwtWindowShow causes the first drawRect -> expose
callback to be fired.