fixed missing textures and dispose() failing on materials

These commits fix two issues: first, a bunch of textures that are included in the sample scenes weren't being tracked for some reason (they contain unicode characters maybe?) so I had to manually add them to the repository.

Secondly, and more importantly, a mesh would fail to dispose() if it had a material that never got assigned a texture. By default, the ._texture property on a material is `null`, not `undefined`, so the `releaseInternalTexture()` function would throw an error whenever disposing of a mesh with a material but no texture. The simple if-null check fixes the issue.