demos: add a fence for presentation

demos: add a fence for presentation

After xglWsiX11QueuePresent(), the image is scheduled for copying.  We should
not render to it until the copying is done.  Otherwise, depending on how the
DDX works, flickering or GPU hangs may be expected.

Add and wait on the fence before submitting the command buffer.  This ensures
the DDX is done with the image.   Set async to true to run as fast as
possible.