Better API for already GPU-resident images

[DJMcNab]

There have been repeated requests for supporting using a wgpu Texture within a Vello Scene, such as #660.

We did implement a targeted fix for this in #636.

In #gpu > vello adding wgpu texture buffers to scene, Chad suggested a new variant of Brush for this use case.
I think that is likely to be the best API, due to the other constraints on the Scene API (namely that we have chosen to make it independent of wgpu, for any consumers who use Vello without using wgpu directly)

[nicoburns]

namely that we have chosen to make it independent of wgpu, for any consumers who use Vello without using wgpu directly

What does this mean? I thought Vello was heavily dependent on wgpu.

[DJMcNab]

We have several users/potential users who cannot use wgpu directly. These include:

1. People who use their own renderer with Vello. See e.g. vello_shaders crate for AOT compilation and lightweight integration with external renderer projects #265, 📀 Make wgpu an optional dependency #359
2. People who want to use Vello with wgpu-core, rather than wgpu (e.g. your colleages https://xi.zulipchat.com/#narrow/stream/197075-gpu/topic/Servo.202D.20canvas.20backend)

[KySpace]

We have several users/potential users who cannot use wgpu directly. These include:

1. People who use their own renderer with Vello. See e.g. [`vello_shaders` crate for AOT compilation and lightweight integration with external renderer projects #265](https://github.com/linebender/vello/pull/265), [📀 Make `wgpu` an optional dependency #359](https://github.com/linebender/vello/pull/359)

2. People who want to use Vello with wgpu-core, rather than wgpu (e.g. your colleages https://xi.zulipchat.com/#narrow/stream/197075-gpu/topic/Servo.202D.20canvas.20backend)

As I am the second kind, what should I do now?