Using another library?
Well, the competency test is clear, it says "Using latest Boost.GIL..." and it says "...using latest Boost.GIL." again
I was thinking of using a Shared Container Iterator
I have never used the shared containers, so no idea what kind of adaptation it would require, but if you are thinking of GIL extension with some custom iterators, then I'd suggest to avoid use of Boost.Iterator stuff (adapter, facade).
I have a dream to rewrite all GIL iterators and get rid of those adapters, see https://lists.boost.org/boost-users/2020/03/90323.php
Before you start asking questions, you need also to read the Boost guides, familiarise yourself with documentation and wiki pages of Boost and Boost.GIL specifically,
the GitHub space, etc.
Before you start asking questions about your project, there is a lot of homework you should do yourself,
nothing to guide you about, as everything can be found online, just do web search