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.