Add functions
   waffle_get_current_display()
   waffle_get_current_window()
   waffle_get_current_context()
and document them in the manpages.

A note about bugs:
  Mixing calls to waffle_make_current() and the native platform's
  MakeCurrent() on the same thread do not interact well.  After calling
  the native platform's MakeCurrent(), future Waffle function calls on
  the same thread are likely to behave incorrectly.

Signed-off-by: Chad Versace <chad.versace@intel.com>