Build Rust stuff offline

[Hofer-Julian]

Fixes #591

cargo-c works fine, but gst-plugin-gtk4 does not
See https://github.com/flathub/io.gitlab.daikhan.stable for existing manifest
See https://github.com/flatpak/flatpak-builder-tools/tree/master/cargo for tool to generate cargo sources

[Hofer-Julian]

I will give up on that, 2h was my personal limit I wanted to spend on it.
If someone else is interested, feel free to pick it up from where I stopped :)

[sonnyp]

Running: cargo cinstall --offline --release --features=wayland,x11glx,x11egl --library-type=cdylib --prefix=/app
Error: CliError { error: Some(no matching package named gdk4-x11 found
location searched: registry crates-io
required by package gst-plugin-gtk4 v0.11.0 (/run/build/gst-plugins-rs)
As a reminder, you're using offline mode (--offline) which can sometimes cause surprising resolution failures, if this error is too confusing you may wish to retry without the offline flag.), exit_code: 101 }
Error: module gst-plugins-rs: Child process exited with code 1

Error: Child process exited with code 1

[sonnyp]

I'll rename ours to gst-plugin-gtk4 too - do some cleanup and merge if you don't mind

Good idea.

Two things that come to mind:

I've used the cargo-c that was used in the manifest, because I thought that might be the reason for the error, might be good to switch to the last release instead and add x-checker
same for gst-plugin-gtk4

The cargo lock file can be created by checking out the tag locally and running cargo update

[sonnyp]

Thanks a lot!

I upgraded cargo-c to the latest release.
I didn't setup x-checker-data because I don't like auto-updates, specially without tests.

We can consider later.