Improvement: Add EWMH _NET_WM_ACTIVE_WINDOW support

- Add support for using _NET_WM_ACTIVE_WINDOW to track focus (rather
  than using FocusIn/Out events) as --use-ewmh-active-win.

- Add wid_get_attr() and use it in place of XGetWindowProperty() in
  various places.

- Remove dpy parameter from find_win() and find_toplevel() as it's unused.

- Fix a writing-to-freed-memory issue in paint_preprocess().

compton.sample.conf

@@ -31,6 +31,7 @@ fade-out-step = 0.03;
 # Other
 mark-wmwin-focused = true;
 mark-ovredir-focused = true;
+use-ewmh-active-win = false;
 detect-rounded-corners = true;
 detect-client-opacity = true;
 refresh-rate = 0;