Bug fix #57: Fix incorrect handling of InputOnly windows

- Stop rendering InputOnly windows. In the past I've misunderstood the
  whole thing, I guess, sorry. Thanks to garfilth and funeral1988 for
  reporting and providing valuable information.

- Fix a possible segfault in DDEBUG_EVENTS.

- Add "dbe" as a configuration file option.

- Attempt to slightly reduce the rendering delay after VSync in non-DBE
  mode. I don't think, however, that this would be greatly helpful for
  the tearing issue.

compton.sample.conf

@@ -32,9 +32,10 @@ fade-out-step = 0.03;
 mark-wmwin-focused = true;
 mark-ovredir-focused = true;
 detect-rounded-corners = true;
-detect-client-opacity = false;
+detect-client-opacity = true;
 refresh-rate = 0;
 vsync = "none";
+dbe = false;
 paint-on-overlay = true;
 sw-opti = false;