Request the dedicated GPU when starting Minetest from the `.desktop` file (#9827)

See <https://www.hadess.net/2020/05/dual-gpu-support-launch-on-discrete-gpu.html>
for more information about this newly added `.desktop` entry property.

misc/net.minetest.minetest.desktop

@@ -11,6 +11,7 @@ Comment[tr]=Tek-Çok oyuncuyla küplerden sonsuz dünyalar inşa et
 Exec=minetest
 Icon=minetest
 Terminal=false
+PrefersNonDefaultGPU=true
 Type=Application
 Categories=Game;Simulation;
 StartupNotify=false