Improvement: GLX: Cache region contents & --glx-no-rebind-pixmap

- Cache region contents in is_region_empty(), mostly useful only for GLX
  backend to save one roundtrip to X.

- GLX backend: Add --glx-no-rebind-pixmap, which prevents rebinding of
  GLX texture to pixmap on content change. It doesn't work on some
  drivers, but it saves some CPU on those where it does.

- Wrap XFree() with a new function cxfree() since its man page claims
  NULL pointers are not acceptable (although in fact it does...).

- Use macro to save some code in get_cfg(). Code clean-up.

src/c2.c

@@ -1183,7 +1183,7 @@ c2_match_once_leaf(session_t *ps, win *w, const c2_l_t *pleaf,
         // Free the string after usage, if necessary
         if (tgt_free) {
           if (C2_L_TATOM == pleaf->type)
-            XFree(tgt_free);
+            cxfree(tgt_free);
           else
             free(tgt_free);
         }