Write forward declarations in another way to avoid
repeated typedefs "error: redefinition of typedef".
Raised when using i686-apple-darwin11-llvm-gcc-4.2
It seems that C apple compiler does not support
C11 feature.
https://bugzilla.gnome.org/show_bug.cgi?id=703885
Before, a gstgldisplay was instancied by the gl src in terms of gl chain.
And then the next element got it through the first gstglbuffer.
Now, this is done though queries.
All glelements get their ref on a gstgldisplay in READY state.
This rewrite is mainly a first step to be able to share OpenGL context hold
by the gstgldisplay using more complex glelements.
For example, with a glvideomixer. The associated gstgldisplay of each gl chain
of the sink pads will share their OpenGL context.
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.
