Use the DPMS extension to detect if screen is turned off, and unredirect
if it is. This also helps working around the problem where OpenGL
buffers lose data when screen is turned off, causing screen to flicker
later when it turns back on if use-damage is enabled.
Unfortunately the DPMS extension doesn't define an event, so we have to
periodically poll the screen state.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
