Add support for background blur

This patch adds a new config option: colors{,2}.blur=no|yes. When
enabled, transparent background are also blurred.

Note that this requires the brand new ext-background-effect-v1
protocol, and specifically, that the compositor implements the blur
effect.

config.h

@@ -192,6 +192,8 @@ struct color_theme {
         bool search_box_match:1;
         uint8_t dim;
     } use_custom;
+
+    bool blur;
 };
 
 enum which_color_theme {