BUG: update RSS extension.

1. fix a bug: the mask image is not init-ed to all 0, which causes problem

2. clean up some debuging code

compare view:

https://github.com/gaoyi/RSSExtension/compare/d447085b3955abb460a8c39e48af925132dd8920…a0260ae4a03c70ba6c3d328d491cfb20afd8e1b1