sparse_strips: render to a `Pixmap` with borrowed data

Nobody has claimed this yet.

From what I could gather, currently, `vello-cpu`'s `Pixmap`s always own their data. In contrast, [`tiny-skia`'s PixmapMut](https://docs.rs/tiny-skia/latest/tiny_skia/struct.PixmapMut.html) allows us to draw directly on borrowed data.

My use-case is that I am using `tiny-skia` to draw my process tree in the background, like so:

The image is redrawn every second, when I go through the `/proc` filesystem to rebuild the tree, and increase the angle by a small amount so that it slowly rotates.

This means I am using `tiny-skia` as essentially a real-time renderer. It always draws to the same (borrowed) underlying buffer, since it can't own the background's byte data.

I would like to use `vello-cpu` instead, since it seems to be more performant than `tiny-skia`. However, as it currently stands, I would have to keep two copies of the whole draw space, and `memcpy` the whole data back and forth. Since I have a 3440x1440 monitor, that would be a total of around 18 MB, which is more than 4x the memory the entire rest of the program is currently using (the background pixels are `mmaped` from a shared memory file only when we need to draw)!

Start by examining how vello-cpu's Pixmap currently owns its pixel data, then compare the borrowed-buffer model of tiny-skia's PixmapMut. Done means vello-cpu can render directly into caller-provided pixel storage without requiring a second full draw-space copy.

Written by the indexing model from the issue text.