#6 Flatpak package; metadata and patches

[Robert McQueen]

I've prepared a cross-distro Flatpak binary package for BillardGL, which, once accepted to Flathub can be installed, securely sandboxed, on any Linux distribution in a single click / command. The policy of Flathub is that ideally, the upstream developers would be responsible for directly publishing/updating their own applications, as then they are able to make decisions around configuration, dependencies, and essentially what is presented to the user and when. If you track the submission at https://github.com/flathub/flathub/pull/1045 then a new GitHub repository will be created for BillardGL and you can get access to it there if you desire.

Worth considering for inclusion directly in the upstream build/install process are these files:
https://github.com/flathub/flathub/blob/fedd31f33b85d8222c675598298792c879ce7272/de.billardgl.Billardgl.appdata.xml
https://github.com/flathub/flathub/blob/fedd31f33b85d8222c675598298792c879ce7272/de.billardgl.Billardgl.desktop
If shipped in /usr/share/meta-info and /usr/share/applications respectively, along with icons in /usr/share/icons/hicolor/SIZExSIZE/apps this wil integrate smoothly with Linux desktops, providing a launchable desktop icon, metadata for software centers, etc.

I have also applied the patch stack from Debian to fix various arithmetic and stability issues, disable fullscreen by defauly, and adjust the build process to handle the different paths inside of a Flatpak sandbox. These patches are available at https://salsa.debian.org/games-team/billard-gl/tree/master/debian/patches and should be considered for merge upstream.