Feature #69: Blur window background

- Add window background blur support (--blur-background &
  --blur-background-frame), with X Render convolution filter.
  The performance sucks. The performance when the window is opaque but
  frame is transparent could be improved, but there are two possible
  ways and I'm hesitating.

- Known issue: The blurring effect looks very ungraceful during fading.
  I could partially fix the problem, but it probably isn't easy to fix
  it completely.

compton.sample.conf

@@ -22,6 +22,8 @@ inactive-opacity-override = false;
 alpha-step = 0.06;
 # inactive-dim = 0.2;
 # inactive-dim-fixed = true;
+# blur-background = true;
+# blur-background-frame = true;
 
 # Fading
 fading = true;