Improvements of the context manager in order to prepare context switching.
Implement WGL context manager
OGLES2 driver was not yet working again, might be a different problem, though. Andoird device was not tested.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4605 dfc29bdd-3216-0410-991c-e03cc46cb475