new backends: blur interface update

To prepare for different blur methods, the blur interface of backends
has been splitted into two parts.

Now to use blur, a blur context must be created first; then, the blur
method should be called with the blur context created.

Updated the existing backends to the new interface. Also implemented
handling of the new blur options.

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

src/common.h

@@ -143,6 +143,9 @@ typedef struct session {
 	ev_signal int_signal;
 	/// backend data
 	backend_t *backend_data;
+	/// backend blur context
+	void *backend_blur_context;
+	/// graphic drivers used
 	enum driver drivers;
 	/// libev mainloop
 	struct ev_loop *loop;