use webp for bttv

[FoseFx]

Fixes #583

Changes

• uses bttv's new webp api

Blocked by #199

Checklist

• My change builds
• I tested my change
• I use the "bttv_" prefix for all resources I propose
• 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
• I'll add myself and everyone else who contributed to this change to the contributors list using all-contributors
• I license my changes according to the MIT License.