Simple implementation split from GstGLWindowWayland
Can now have multiple glimagesink elements all displaying output
linked via GL or otherwise (barring GL platform limitations).
The intel driver is racy and can crash setting up the two glimagesink contexts.
e.g.
videotestsrc ! tee name=t ! queue ! glupload ! glimagesinkelement
t. ! queue ! gleffects_blur ! glimagesinkelement
videotestsrc ! glupload ! glfiltercube ! tee name=t ! queue ! glimagesinkelement
t. ! queue ! gleffects_blur ! glimagesinkelement
