Enable Wayland support in Qt, use it if available

[smcv]

• Compile native Wayland support plugin for Qt

  We'll use this to give the UI shell Wayland support.

• Revert "Fixed bug Force Xwayland on Wayland #6, explicitly set the QT platform for the shell"

  Now that we have Qt's platform plugin for Wayland, we can use that
  for the shell. This is a step towards not needing X11 at all.

  Test with:

    Xephyr :2 &
    DISPLAY=:2 flatpak run com.valvesoftware.SteamLink//test
  

  Previously, both the shell (UI layer) and the actual streaming client
  would run on the Xephyr display. After this change, the shell runs
  full-screen again (via Wayland), and the actual streaming client uses
  X11 by default.

  The streaming client can be forced to use Wayland too by setting
  environment variable SDL_VIDEODRIVER=wayland.

  This reverts commit 9845f4f.

---

@lhindir, please could you try this out in your Wayland environment? Let us know whether it works well, what your Wayland environment is, and which Qt and SDL environment variables you're forcing (if any).

It seemed OK for me in some brief testing in GNOME Shell 3.38, but I haven't tried it in other Wayland environments like KDE Plasma or Sway.

[flathubbot]

Started test build 44786

[flathubbot]

Build 44786 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/43087/com.valvesoftware.SteamLink.flatpakref

[lhindir]

Thanks @smcv, I will get to this soon.

[flathubbot]

Started test build 47519

[flathubbot]

Build 47519 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/45698/com.valvesoftware.SteamLink.flatpakref

[smcv]

In addition to GNOME this seems to work nicely on KDE Plasma in Wayland mode (Debian 11's plasma-workspace-wayland package, on AMD hardware) and on Weston, so I'm going to try landing this in the beta.