Improvement: Improve color inversion performance & Validate pixmap

- Try to improve the performance of color inversion by applying clipping
  region during color inversion. (#75)

- Validate pixmap on window unmap/destruction. Probably slightly helpful
  for #52.

- Change the design of unmap_win() and destroy_win(), a bit.

- Add warning message to help messages about features disabled at
  compile time, instead of dropping their description completely. (#85)

- Silence some warnings. Code clean-up.