More efficient compression formats (WebP / AVIF)

[heyakyra]

The times are a changin'!

Snap.as/SnapFreely could offer higher quality for the same file sizes if adopting newer formats.

• PNG / BMP → WebP
• Photos (Jpeg / RAW / etc) → AVIF

Discussion on forums: https://discuss.write.as/t/snap-as-support-for-webp-images/1464/2