use webp for bttv

[FoseFx]

Fixes #583

Changes

• uses bttv's new webp api

Blocked by #199

Checklist

• [x] My change builds
• [x] I tested my change
• [x] I use the "bttv_" prefix for all resources I propose
• [x] When adding a string I also added it to the bttv.Res.strings Enum and res/values/strings.xml (in mod) and res/values/public.xml (in disass)
• [ ] If my change is significant enough, I added it to the CHANGELOG.md under master
• [x] I'll add myself and everyone else who contributed to this change to the contributors list using all-contributors
• [x] I license my changes according to the MIT License.