Unify instantation of image handles with ImageFileNameProvider

[akoch-yatta]

Currently there is an inconsistent behavior how files retrieved via ImageFileNameHandler are converted into handles. Currently it is as follows:

Ask ImageFileNameHandler for file for targetZoom

• If perfect match -> call initNative(...)
• If no perfect match -> call new ImageData(filename) -> this breaks with some files, e.g.
  tiff_image.zip

We need to unify this by always calling initNative first to create a scaled variant of it on demand if necessary. Therefor we need to refactor initNative to serve all required use cases.