epaint (and render backends) as a standalone crate.

**Is your feature request related to a problem? Please describe.**

The tessellator in epaint is really good and I would like to use it as a library in another project. That is quite possible right now, but to alto get the render backend, e.g. egui-wgpu, you have to pull in egui as well. That in itself is not a huge issue, but unnecessary.

The only dependency on egui, for egui-wgpu at least, is to use the ViewportId for multi-window support.

**Describe the solution you'd like**
Removal of the dependency to egui on egui-wgpu and maybe egui-glow.

ViewportId is only used as the key in a hashmap, so there is no special need for the full type. It could be reduced to just storing the NonZeroU64 type that it contains within?

**Describe alternatives you've considered**
- Just dealing the the egui dependency, which isn't bad, but projects bloat up so quick, its nice to keep things to a minimal.
- Using another library.

**Additional context**

Start by tracing how egui-wgpu and egui-glow use ViewportId, especially its role as a hashmap key, and review how epaint is currently exposed. Check the existing backend builds or tests, then verify that the render backends no longer depend on egui while multi-window support still works and epaint can be consumed independently.

Written by the indexing model from the issue text.