Add the vello_toy crate

[LaurenzV]

This PR proposes adding a new crate called vello_toy, containing a number of small binary crates that build on top of the other ones. Currently, it only contains a single tool debug that allows to visualize the intermediate stages of sparse strip rendering, but I think there are a couple of other binary crates that could be added in the future (another idea of mine being for example building a very simplistic SVG renderer on top of usvg).

The current tool I added serves two purposes:

• It allows newcomers to experiment with and visualize the various stages, which makes it easier to understand how everything works.
• It allows for debugging issues in specific parts of the pipelines more easily.

I know there could be more documentation on what exactly each of the parameters mean and how to use them, but since it's mainly for development purposes I think this should be enough. To demonstrate this with an example:

cargo run --bin debug -- --path "M 5 5 L 40 23 L 7 44 Z"  --stages ls,ta,ti

cargo run --bin debug -- --path "M 5 5 L 40 23 L 7 44 Z"  --stages ls,ta,ti,sa,s

cargo run --bin debug -- --path "M 5 5 L 40 23 L 7 44 Z"  --stages ls,ta,ti,sa,s,wt

[LaurenzV]

If we don't want to create a separate crate I'm open for that, but I'm not sure if something like this is worth putting in examples, maybe it is? Another disadvantage of that would be that vello_common would require a dev dependency on clap and svg.

[grebmeg]

That sounds like a great idea! I really liked your debug example in the cpu-sparse-experiments repo and actually started building a web debugger on top of it (attached a video). I wanted to share it earlier but didn’t get the chance. Initially, I thought it might be a good fit as one of the examples. What do you think about the web debugger? Would it be useful for you as well?

sparse-strip-web-debugger.mov

[LaurenzV]

Ohh yeah, this looks pretty neat! :) Maybe that would be a better way of implementing this, since it's interactive.