Replace vello's default wgpu feature with an optional wgpu feature

[MarijnS95]

linebender/vello#359 made wgpu an optional but default dependency/feature, but any crate using vello_svg will be forced to opt in to wgpu due to not disabling the default here.

Since vello_svg does not need any rendering backend at all, this feature should be disabled by default. As the vello crate is reexported from the root, a vello-wgpu passthrough feature is provided so that users can turn it back on without explicitly depending on vello in their crate dependencies.

Same change as linebender/velato#17.

[simbleau]

Can you explain why we wouldn't want the default feature for wgpu?

[MarijnS95]

So that crates using vello with their own rendering backend - without wgpu - can use the SVG implementation provided here, without pulling that dependency in? Same reasoning as linebender/vello#359 that did it for the vello crate.

[simbleau]

So that crates using vello with their own rendering backend - without wgpu - can use the SVG implementation provided here, without pulling that dependency in? Same reasoning as linebender/vello#359 that did it for the vello crate.

Ah, I see.

We actually don't need the wgpu feature at all, I think. Since this crate only renders a Scene. This makes sense to me then.

[simbleau]

Please update the CHANGELOG in turn and I can approve this. Thanks!

[MarijnS95]

We actually don't need the wgpu feature at all, I think. Since this crate only renders a Scene. This makes sense to me then.

Yes, I could have spent a few more words explaining that vello_svg (and velato for the other PR) don't expose/use any of the render-backend specific implementation.

---

Before pushing an updated changelog: since this crate reexports vello from the root, does it make sense to forward the wgpu feature by adding wgpu = ["vello/wgpu"]?

[simbleau]

Yeah, let's do that.

[MarijnS95]

@simbleau done.

[simbleau]

Let's change that feature name to just wgpu.

[simbleau]

Thanks!