Pass $meta_id into sanitize_meta() and subsequently into the filter for register_meta callback

[sc0ttkclark]

To use Post Type based sanitization for specific meta keys, currently register_meta adds the filter but while the 'auth' filter gets access to the Post ID, the 'sanitize' filter does not. We would need to add $object_id in the cases of add/update_metadata, and in the case of update_metadata_by_mid we'd need to pass the Post ID into that function as well, which would then pass along to sanitize_meta() usage.

Is anyone against this proposal? If so, a patch can be drawn up.

[jrf]

Makes sense to me and I see no issues with backward compatibility.
Only issue would be direct calls to sanitize_meta(), but plugins should hook into the filter, so I don't actually expect an issue with that.

Added a patch implementing @sc0ttkclark's suggestion.