Should compile under Windows again

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4595 dfc29bdd-3216-0410-991c-e03cc46cb475
Abstract away the EGLManager into IContextManager and add the manager for win32/ogles1 as well.
Android device should still compile and work, but was not tested.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4596 dfc29bdd-3216-0410-991c-e03cc46cb475