Connect to tracy if the tracy feature is enabled

[DJMcNab]

This is very useful for debugging performance issues. I've already used it to debug #masonry>to_do_list: Horrendous performance.

Setting a feature works on a workspace-wide basis. I.e cargo run --example mason --features tracy works.

Some weirdness:

• Tracy seems to assume we know when a frame will "start", but we only know if we'll repaint once processing for an event has finished.
• Something seems to be starting a default "Frame", which isn't really right. We don't have any continuous frames. Maybe we should always request a redraw if tracy is enabled.

Either way, I think this feature is useful to land now, so am not planning on resolving these in this PR unless someone provides a solution.

[PoignardAzur]

The code isn't too clear if you don't know wgpu_profiler or tracy, but it's mostly plumbing anyway.

I would like a short comment in the code (not just the Cargo manifest) explaining what tracy is, and giving a short explanation of how to use it.

Ideally we should have a full "How to debug Masonry perf problems" tutorial somewhere in our documentation, but that's a future item.