doc/vision.md

[ratmice]

Being somewhat late to vello, having not played with it during the piet-gpu stages of development.
It reads to me like doc/vision.md is somewhat out of date with the current state of things,
In particular the section Why not wgpu doesn't seem relevant anymore since vello does use wgpu.

So that could perhaps use a pass over it to better reflect the current state of things.

[PoignardAzur]

See #488