Runtime Virtual Textures

[drichardson]

Add entry for Runtime Virtual Texture Assets.

In spite of the name, URuntimeVirtualTexture is a sub-class of UObject, not UTexture. Therefore, it probably should not be prefixed with T_, since it cannot be used where textures are normally used.

I propose using RVT as the prefix, as Runtime Virtual Textures are abbreviated like this in the documentation.

Assuming this is good with anyone who cares, I'll have a PR for the change in a bit.