Support webp image format for screenshots and other images

[hsandt]

Is your feature request related to a problem? Please describe.

This is a ticket for the itch threads:

• https://itch.io/t/1353417/add-support-for-webp-image-format (on which admin agreed, but is already 3 years old)
• https://itch.io/t/2342680/please-add-webp-image-support-for-screenshots-and-other-images

Webp images are not supported for screenshots (and other image attachments, banners, etc.) but they would greatly reduce file size while keeping good quality.

Most of my screenshots are in webp, to upload them, I have to convert them to either:

• PNG: this is huge esp. for an HD game. Even for pixel art games, PNG scale with window scaling, while webp is more efficient and will keep a very small size on pixel art scaling - and a small size on HD pictures
• JPG: this is better but artifacts tend to show faster as quality decreases, and still takes 2x space of webp

Size comparison example for an HD game, 1080p screenshots:

• PNG: 2 MB
• JPG quality 90: 430 kB
• webp quality 90: 250 kB

Describe the solution you'd like
Support webp images for screenshot, image attachments, promotional material

Describe alternatives you've considered
I must convert to JPG but that's still bigger in size and not convenient for me to duplicate all my screenshots.

Additional context
Applies to both HD and pixel art games.

[AdamPype]

most importantly: webp supports gif-like animation with a small file size, which is great because it's really difficult to make good gifs for the game thumbnails especially, and screenshots, that fit in the small file size