Canon CR3 HEIF previews extraction with libraw 0.21.0

[Gilles Caulier]

Hi all,

Canon CR3 users can complain that previews extracted from Raw files
cannot be displayed. In fact Canon uses now HEIF to store preview
instead of JPEG.

With last libraw 0.21.0 that i update in digiKam core today,

https://www.libraw.org/news/libraw-202101-snapshot

...we have a new flag to indicate that preview data are encoded with H265:

Canon CR3 H265 (HEIF) preview limited support:
- new format value: LIBRAW_THUMBNAIL_H265
- this thumbnail is NOT extracted by LibRaw::make_mem_thumb() yet;

So, we need to plan a decoded workflow with libheif with the
corresponding byte-array.

Best

Gilles Caulier