Use temp file to save metadata for readonly file

I hadn't spotted this before, but we have created a naming problem here. I can't imagine why _pathForSavingMetadataChanges was ever static, but it used to be mutated only when you created a new image. Now, when we switch to a temp file, we're updating it. At that point it is hardly the 'OriginalFilePath' any more.
I don't know whether anything actually wants the OriginalFilePath, but I think _pathForSavingMetadataChanges should be replaced with a simple property PathForSavingMetaDataChanges, and if we also have a use for OriginalFilePath, that should be an independent (but still non-static) property initialized at construction and never changed.
Sorry I didn't spot this earlier...