Fix assetName for textures when the parent ldtk is in a nested folder

[jaguire]

The path in the ldtk file is relative to itself, but the asset name is a path relative to the content folder. Since larger projects will have a folder structure under the content folder. The content.Load<T> needs to respect that structure. This gets the folder(s) in between content and the ldtk file and prepends it to the filename. It just passes the file through if in the root.

Content
  ├─ LDtkFiles
  │    ├─Atlas
  │    │    └─ Cavernas_by_Adam_Saltsman.png
  │    └─ AutoLayers_1_basic.ldtk
  ├─ Images
  │    └─ ...
  └─ Fonts
       └─ ...

This fixes: content.Load<Texture2D>("Atlas/Cavernas_by_Adam_Saltsman")
To: content.Load<Texture2D>("LDtkFiles/Atlas/Cavernas_by_Adam_Saltsman")

[IrishBruse]

Thanks for the PR in the meantime you can use your own renderer to fix this just copy the LDtkRenderer into your project and use that one instead when I get a chance ill make a release.