Only supported on Win32 and SDL devices so far (X11 should get support for it soon).
This was mainly to work around troubles with SDL+OpenGL on some platforms where resizing after window creation can be tricky/impossible.
See http://irrlicht.sourceforge.net/forum/viewtopic.php?f=7&t=52083 for discussion.
Removing the hack with shared GL resources on Windows in SDL device setResizable.
For some reason that hack worked in 1.8, but fails now. Not sure why, but as SDL1 device will likely be deprecated and replaced by SDL2 soon I won't 
investigate that further for now. With the new WindowResizable it's at least possible to get a resizable SDL window now (thought on some platforms even resizing may drop
GL resources in which case it will still fail).


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@5871 dfc29bdd-3216-0410-991c-e03cc46cb475