Improvement: Add painting on overlay support

- Add support for painting on X Composite overlay window instead of root
  window (--paint-on-overlay). I intended to use this to fix the
  conflict between conky (own-window off) and compton, but it's
  unsuccessful. Will have to ask somebody to figure out how to solve
  this problem.

- Rename a few variables to avoid confusion.

- Slightly change how root window content (wallpaper) change is
  detected.

- Slightly improve window name detection in DEBUG_EVENTS.

compton.sample.conf

@@ -34,6 +34,7 @@ mark-ovredir-focused = true;
 detect-rounded-corners = true;
 detect-client-opacity = false;
 refresh-rate = 0;
+paint-on-overlay = true;
 
 # Window type settings
 wintypes: