Unlit Render Mode / Batching shader doesn't compile on some devices

Linking issue from Hubs: https://github.com/mozilla/hubs/issues/1831

We may want to consider drawing attention to this in some way to make it more clear that there are alternate render modes, or choose a default based on device capabilities?

Start by reviewing the linked Hubs issue and tracing where the Unlit Render Mode and batching shader are selected and compiled. Compare behavior across affected devices, then determine whether alternate render modes need clearer visibility or whether a device-capability-based default is appropriate. Done means the shader compiles on affected devices or users are directed to a compatible render mode.

Written by the indexing model from the issue text.