Fixes bug #584877
Before this commit calling "gst_x_overlay_set_xwindow_id" more
than one time, had no effect.
It mainly affects the glimagesink implementation.
But on win32 (and CE), some stuff has to be done to
release the old parent.
And add a switchxoverlay example where the user
can click on left/right part of the main window to
switch the xoverlay.
The external opengl context must be specify when creating
our OpenGL context (glx) or just after (wgl).
When calling glXCreateContext or wglShareLists, the
external opengl context must not be current.
Then our gl context can be current in the gl thread while
the external gl context is current in an other thread.
See tests/examples/clutter/cluttershare.c
In OpenGL 2.x for Embedded System, a lot of basic scene/draw functions
have been removed. It means that everything is made using vertex and
fragment shaders.
I have also added a gstglwindow backend for winCE that uses EGL
(Native Platform Graphics Intercace) (which is a full part of
OpenGL ES specification). It remove the use of wgl/glx functions.
