SymbolName limited to vispy marker types

[gselzer]

As written in #65:

@tlambert03, since you wrote scenex.model.SymbolName: there are many symbol types that pertain only to pygfx that aren't in that list, and conversely some symbols that the pygfx backend can't (without a custom marker type, which is some additional effort) handle.

There are a few ways that we could proceed:

1. Add pygfx-only types to SymbolName - works, but could drastically expand the size of the Literal, especially if more backends are added. Don't know if this is a problem, though.
2. Switch to a SymbolName | str, which would keep SymbolName as a common subset but allow each backend to support its own styles as well.

[tlambert03]

don't have a strong opinion here. SymbolName | str seems the most honest... failing at runtime with a specific backend is fine