Misc: Try to avoid evaluating conditions after window unmap & others

- Try to avoid evaluating conditions after window unmap/destruction.
  Unfortunately, frequently this doesn't work due to the asynchronous
  nature of X.

- Add _GTK_FRAME_EXTENTS exclusion rules to compton.sample.conf. Thanks
  to memeplex, hexchain, and others for info. (#189)

- Add debugging option --show-all-xerrors, and other debugging changes.
  Doc update.

src/c2.c

@@ -1292,6 +1292,8 @@ c2_match_once(session_t *ps, win *w, const c2_ptr_t cond) {
 bool
 c2_matchd(session_t *ps, win *w, const c2_lptr_t *condlst,
     const c2_lptr_t **cache, void **pdata) {
+  assert(IsViewable == w->a.map_state);
+
   // Check if the cached entry matches firstly
   if (cache && *cache && c2_match_once(ps, w, (*cache)->ptr)) {
     if (pdata)