Flatpak doesn't allow automatic branch following for extension

[hfiguiere]

Linux distribution and version

Fedora 33

Flatpak version

1.10.2

Description of the problem

Flatpak doesn't automatically update the extensions in their branch change at an update.

Steps to reproduce

• An application (org.gimp.GIMP in that case) has an extension point defined with the version field set to 2-3.36.
• This application gets updated to use the the GNOME 40 runtime. The version field for the extension point is changed to 2-40 because the runtime content is different and we assume the runtime are incompatible
• The extensions org.gimp.GIMP.Plugins.* are updated to be on the branch 2-40
• Everything is build
• The user update org.gimp.GIMP

Expected

• Flatpak notice a change in the extension version and will trigger an update of already installed extensions

Actual

• Nothing else is updated and the user lost the plugins in GIMP.

[soredake]

It'll be nice if we can somehow link extensions/layers version to app versions to avoid situations like this flathub/org.freedesktop.Platform.VulkanLayer.MangoHud#17