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/backend/gl/glx.c

@@ -472,6 +472,8 @@ struct backend_operations glx_ops = {
     .buffer_age = glx_buffer_age,
     .render_shadow = default_backend_render_shadow,
     .fill = gl_fill,
+    .create_blur_context = gl_create_blur_context,
+    .destroy_blur_context = gl_destroy_blur_context,
     .max_buffer_age = 5,        // Why?
 };