gl/eagl: don't access UIkit objects on the main thread

gl/eagl: don't access UIkit objects on the main thread

This means we cannot access [view layer] or view.bounds from the OpenGL
thread.  This also means that we need to call the main thread when
setting the window handle.  However, we cannot perform that
synchronously as that may deadlock with the application performing the
set_window_handle() call.

We need to defer the actual update and run it asynchronously and wait
for the window handle update internally at each point it is needed.

Fixes: https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/372
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/681>