MetaRendererNative: Flush all pending swap notifies on idle

We need to do swap notifications asynchronously from flip events since
these might be processed during swap buffers if we are waiting for the
previous frame's flip to continue with the current.

This means that we might have more than one swap notification queued
to be delivered when the idle handler runs. In that case we must
deliver all notifications for which we've already seen a flip event.

Failing to do so means that if a new frame, that only swaps buffers on
such a swap notification backlogged Onscreen, is started, when later
we get its flip event, we'd notify only an old frame which would hit
this MetaStageNative's frame_cb() early exit:

  if (global_frame_counter <= presented_frame_counter)
    return;

and we'd never finish the new frame and thus clutter's master clock
would be waiting forever stuck.

https://bugzilla.gnome.org/show_bug.cgi?id=774557