Improvement: Dynamic blur strength & improved frame blur performance

- Remove the region expansion design in win_blur_background(). I must be
  sleep-walking when I wrote that!

- Improve performance of blurring when a window is opaque but its frame
  is transparent.

- Adjust blur strength according to window opacity.
  --blur-background-fixed restores the old behavior.

- Add "use_offset" parameter to a few functions for convenience. Code
  clean-up.

compton.sample.conf

@@ -24,6 +24,7 @@ alpha-step = 0.06;
 # inactive-dim-fixed = true;
 # blur-background = true;
 # blur-background-frame = true;
+blur-background-fixed = false;
 
 # Fading
 fading = true;