Refactor SkJpegXmp into SkXmp which is more generic.

Useful for formats other than jpeg that would use the same
gainmap xmp format for example.

Change-Id: Id1e69032a4566e96fdc4d0038ee6a6b39ea7ed95
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/699656
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
16 files changed