scarab: Apply scaling factor in Wayland

Sets `AVALONIA_GLOBAL_SCALE_FACTOR` to the GNOME desktop scaling factor
based on
<AvaloniaUI/Avalonia#9390 (comment)>,
falling back to the X FreeType DPI setting if the former is not
available.

pkgs/tools/games/scarab/default.nix

@@ -2,12 +2,15 @@
   lib,
   buildDotnetModule,
   fetchFromGitHub,
+  bc,
+  glib,
   glibc,
   zlib,
   gtk3,
   copyDesktopItems,
   icoutils,
   wrapGAppsHook3,
+  writeText,
   makeDesktopItem,
 }:
 
@@ -34,6 +37,8 @@ buildDotnetModule rec {
   '';
 
   runtimeDeps = [
+    bc
+    glib
     glibc
     zlib
     gtk3
@@ -60,6 +65,8 @@ buildDotnetModule rec {
       size=''${sizes[$i]}x''${sizes[$i]}
       install -D omegamaggotprime_''$((i+1))_''${size}x32.png $out/share/icons/hicolor/$size/apps/scarab.png
     done
+
+    wrapProgram "$out/bin/Scarab" --run '. ${./scaling-settings.bash}'
   '';
 
   desktopItems = [

pkgs/tools/games/scarab/scaling-settings.bash

@@ -0,0 +1,17 @@
+# Keep existing value if it is already non-empty
+if [[ -z "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]]; then
+    # Attempt to get GNOME desktop interface scaling factor
+    AVALONIA_GLOBAL_SCALE_FACTOR="$(gsettings get org.gnome.desktop.interface scaling-factor | cut --delimiter=' ' --fields=2)"
+fi
+
+if [[ -z "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]]; then
+    # Attempt to get scaling factor from X FreeType DPI setting
+    dpi="$(xrdb -get Xft.dpi)"
+    if [[ -n "$dpi" ]]; then
+        AVALONIA_GLOBAL_SCALE_FACTOR=$(echo "scale=2; $dpi/96" | bc)
+    fi
+fi
+
+if [[ -n "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]]; then
+    export AVALONIA_GLOBAL_SCALE_FACTOR
+fi