compose: Provide a switch to use absolute URLs instead of media_baseurl

[barthalion]

A comment in the neighbouring repository mentions a knob that would allow avoiding media_baseurl when --media-baseurl and --media-dir are used. We would like to use that for Flathub to keep backwards compatibility with distributions using too old appstream-glib/libappstream to understand media_baseurl attribute, but it's at least undocumented, or not available with CLI.

[razzeee]

FYI, this is one of the issues we need to have solved to migrate flathub (again)

[ximion]

Sorry, I had a bit too much to do the past months, currently working through the task backlog.

Looks like this feature is actually implemented, but completely unusable / broken (as most people don't seem to want their software stores to callout to random websites).

I'll have a look if there's a sensible way to fix this :-)

[ximion]

This patch ^ adds this feature to compose.
I also backported it to the stable series, at it is fairly small and self-contained (even though it's a new feature).