missing PBR (Physically Based Rendering) part

Hi,

First of all thaks for creating this amazing library. It really helps me to build things. Could you please exaplain why the Physically Based Rendering was removed as the previous versions of this library were handling that?

For example:

```C#
Material.PBR.HasTextureBaseColor
Material.PBR.HasTextureNormalCamera
Material.PBR.HasTextureMetalness
Material.PBR.TextureMetalness
//------
Material.IsPBRMaterial

```
and so on. Why those were removed? The library is not handling PBR any more?

The issue names the missing Material.PBR and Material.IsPBRMaterial APIs but no source files or tests. Compare the current material API with the previous library versions, starting from those names, to determine whether PBR support was intentionally removed. Done would require an agreed scope for restoring the requested PBR properties and corresponding validation.

Written by the indexing model from the issue text.