semi-transparent

Preface

Due to Add support of animated sticker #78

There is currently no direct way to convert WebM to GIF this on the Internet.

My idea is: WebM -> WebP -> GIF (due each steps have existing tools)

Here are the difficulties we need to overcome:

1. Remove vp8/9 from WebM to let we more easy to do next (Refer How can I convert WebM file to WebP file with transparency?)
2. Convert WebP to GIF (Refer How to Convert WebP image to Gif with Python?) and handling semi-transparent pixels (here is what we need to face)