This really feels like internal API which we'd want to keep private (and
used by core plug-ins only).
Also as Jacob noticed, it's not even included in libgimpbase/gimpbase.h
so plug-ins wishing to use this API need to include this file
specifically anyway (but the header is still installed and the API is
introspected).
Since we cannot remove these functions now that GIMP 3 was published,
for API stability, I am only deprecating them both in the C API with
macros and in the bindings with GObject Introspection annotations.
Therefore any third-party plug-in developer trying to use these
functions in a plug-in will get build-time or run-time warnings.
Then when we'll move on to GIMP 4 development, we can remove the
deprecation and simply make this file private-only use instead.
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