Investigate memory usage of backends

With the `winit` example in this repo (and the default 800x600 logical size (with 2x scale factor)) get:

- `61792` bytes using a new `null` backend (which doesn't do any rendering at all and has a memory usage of 1 byte)
- `85728` bytes using the `vello_cpu`+`pixels` backend
- `95920` bytes using the `vello_cpu`+`softbuffer` backend
- `86544` bytes using the `vello` backend

In all cases (except the null backend) memory usage jumps up quite a bit if the window is resized bigger. To ~100mb for `vello` and ~120mb for `vello_cpu`+`pixels` with a fullscreen window on my MacBook Pro 14".