Make DNG OpcodeLists available from rawler

[paolodepetrillo]

The purpose of this change is to make the DNG OpcodeList tags available through the rawler API, so that they can be accessed from within vkdt to implement some opcode list processing for lens corrections.

It specifically gets the tags corresponding to the raw image, which might be in either Image or SubImage1, and ignores any OpcodeLists for preview / thumbnail images elsewhere in the file.

I also started writing a decoder in rust for the opcodes - can finish it and provide it if it would be useful for implementing opcode list processing in dnglab in the future.

[cytrinox]

Thanks for your first contribution.

As DNG opcodes are not related to EXIF but to RAW metadata, these new properties should moved into RawMetadata struct instead.

Opcode processing would be awesome. I've yet added a new raw development pipeline to generate previews (if missing) and long-term idea is to allow preprocessing raw data before writing into DNG like scaling, demosaic etc. Both processes would benefit from opcode support.

[paolodepetrillo]

I will open another PR that puts the properties directly into RawMetadata.

A while ago I had made this simple script dngpreprocess which used the Adobe DNG SDK to apply the OpcodeList2 / 3 processing and write out a DNG. The purpose was a short term workaround to allow DNGs that needed the opcode list processing to be edited in apps like darktable that didn't have full opcode support yet. Eventually if the opcode processing is added to dnglab, it could replace that script.