fix: importing props/mounts in projects

[Bronya-Rand]

This addresses two issues reported by Holo in Aetherworks where the Shader code would apply to props and break mounts during a project import.

The main cause of the issue was that props/mounts don't really have a extended appearance and thus:

1. If the actorFile is a prop, the shader code applies and defaults to (what I assume is your actual character) a generic character appearing alongside the prop itself.
2. When saving a project, the ShaderParams argurment is required which can cause saving issues for props/mounts.

For 1, all that needed to be done is expand the if calls for props and add a fallback result for chars with bad shaders. For 2, I made the argument for shaders in the ActorExtendedAppearance optional for.