Refactor Brush to use generic types for image and gradient data

[sagudev]

As discussed in #133 (review) we can also generalize Brush over image data and gradient. This makes BrushRef<'a> = Brush<ImageBrushRef<'a>, &'a Gradient> which allows to share more From impls, most notably various color implementations (even owned version can now support conversion from &color because color is copy anyway).

[sagudev]

I think we want this for vello_cpu so we can remove: https://github.com/linebender/vello/blob/0640012afc5a5454644ca51f13ee4128529c5ffd/sparse_strips/vello_common/src/paint.rs#L82

cc @LaurenzV

[LaurenzV]

@DJMcNab Is this intended to land before 0.5.0?

[DJMcNab]

We talked about this in office hours yesterday. My thinking is to not do so - this is something which it only makes sense to think about once we get Vello API working, probably.

If this would be useful for Vello CPU, it could land though. I don't think it has a huge cost.

[sagudev]

This change is backwards compatible, so it can be done in patch release.

If this would be useful for Vello CPU, it could land though. I don't think it has a huge cost.

My idea is to start passing this type down to vello_cpu renderer (instead of own type) and deal with opacity, BGRA and alpha on sampling, but we could always just continue to roll own sparse strips types (and extend them) like we do now.

[LaurenzV]

Maybe I am missing something obvious, but how is it backwards-compatible? The PR introduces generic parameters to a public enum, which seems like a pretty breaking change to me, no?" 🤔

[sagudev]

Maybe I am missing something obvious, but how is it backwards-compatible? The PR introduces generic parameters to a public enum, which seems like a pretty breaking change to me, no?" 🤔

Generic parameter have default values that are same as before, so they are backwards compatible AFAIK.

[LaurenzV]

Ahh okay, fair enough, I missed that.

[LaurenzV]

I mean to me, that seems reasonable, but I think it might be a good idea to get a draft PR in vello_cpu up working before we actually merge this, to make sure it works as intended.

[sagudev]

Hm, I just rebased and docs are failing.

[DJMcNab]

I think this now blocks #140, because I do suspect this is probably still breaking, but only in niche scenarios.
The changes look fine to me

I'm happy enough keeping this out of the changelog; it's not a functionality I particularly want to advertise, because it's a bit messy for our interstitial state.

[nicoburns]

Ah, it hadn't occured to me that this would enable Brush to be used in sparse strips vello. This is now making sense to me.

[sagudev]

Ah, it hadn't occured to me that this would enable Brush to be used in sparse strips vello. This is now making sense to me.

This is my main motivation.

Given that CI passes in linebender/vello#1253 and the amount of support for this I am going to land this (so we can unblock release).