Reading the Exif in the load background thread

**Glide Version**: 4.8.0

Since Glide doesn’t have Exif support for **raw (dng)** image files, we are currently using the **Exif Support Library** to read the correct orientation for such format and supply a custom transform class according to it.

The problem is that reading the Exif in the UI thread right before committing to Glide the task to load the image has a performance hit.

If Exif support for such format isn’t something in the roadmap, then, is there any callback in the load pipeline triggered in the **loading background thread** that we could use read the Exif orientation and supply the transform to be applied?