Assuming you know the given onscreen framebuffer is based on an x11 window this queries the XID of that window.
If cogl_x11_onscreen_set_foreign_window_xid was previously called then it will return that same XID otherwise it will be
the XID of a window Cogl created internally. If the window has not been allocated yet and a foreign xid has not been set then it's
undefined what value will be returned.
It's undefined what this function does if called when not using an x11 based renderer.
| onscreen |
A Onscreen framebuffer |