Downscale pasted PNG images based on metadata (#29123)

Downscale pasted PNG images based on metadata (#29123)

Some images like MacOS screenshots contain
[pHYs](http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8)
data which we can use to downscale uploaded images so they render in the
same dppx ratio in which they were taken.

Before:

<img width="584" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/50979e3a-5d5a-40dc-a0a4-36eb6e28f14a">

After:

<img width="329" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/0690902a-f2fe-4c6b-97b3-6fdd67c21bad">