Support all alpha values

Remove alpha_step, and support all 256 alpha values.

1 pixel pictures don't really use that many resources.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

compton.sample.conf

@@ -27,7 +27,6 @@ inactive-opacity = 0.8;
 # active-opacity = 0.8;
 frame-opacity = 0.7;
 inactive-opacity-override = false;
-alpha-step = 0.06;
 # inactive-dim = 0.2;
 # inactive-dim-fixed = true;
 # blur-background = true;