z indexing plugins

We need to design an explicit solution for broadcasters to chose how their loaded plugins render (by z index) so that you can stack plugins as needed. Perhaps this is advocating for layers.

For instance, imagine a webcam plugin, and some webcam chrome. Broadcasters would expect to be able to layer this content, as they can in OBS today.

No files or tests are named. Start by locating the plugin-loading and overlay-rendering entry points, then read how multiple plugins are currently composed. Define the expected ordering behavior for cases such as a webcam plugin and webcam chrome; done means broadcasters can control plugin stacking explicitly and the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.