Add IMB_ispic_type_from_memory so we can detect the format of a packed file.

This removes is_a_filepath callback and uses a magic check for photoshop files
that's compatible with OIIO.

This means we can know the file format without having the file on-disk (packed files) for e.g.

Even though OIIO doesn't support packed images, this doesn't have to prevent us from being able to check the file-type of in-memory data.

To cover points from https://wiki.blender.org/wiki/Process/Contributing_Code#Ingredients_of_a_Patch

• Description of the problem that is addressed in the patch.
  
  Currently we can't know the format of a packed image.

• Description of the proposed solution, and a motivation as to why this is the best solution.
  
  Use is_a callbacks for all image types (this was only missing for photoshop files)

• List of alternative solutions, and a motivation as to why these are undesirable.
  
  D9500: Add file extension to unpacked image could be fixed differently, for example the file-path of generated image files could always be set on packing, but this only solves the issue for newly packed files.

• Limitations of the proposed solution.
  
  None, it's using existing API's in a more consistent way.