This pull request removes more `Send` bounds from GPUI2 after #3206 and
simplifies some internals. Specifically:

- The `Reference` enum was removed, as we always capture mutable
references anyway.
- A few GATs from `Context` and `VisualContext` were removed, as they're
unnecessary now that `MainThread` isn't a thing.
- View rendering was greatly simplified (we were able to remove
`EraseViewState` and `ViewObject`)

Release Notes:

- N/A