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.

Does not include `gsettings` and `xrdb` in build inputs, since these
should be available on the relevant platforms.

Bash does not support decimal/floating point arithmetic, so this *does*
include `bc` in the runtime dependencies.

pkgs/tools/games/scarab/default.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  bc,
   buildDotnetModule,
   fetchFromGitHub,
   copyDesktopItems,
@@ -29,6 +30,10 @@ buildDotnetModule rec {
       -n nuget.org --configfile NuGet.Config
   '';
 
+  runtimeDeps = [
+    bc
+  ];
+
   nativeBuildInputs = [
     copyDesktopItems
     icoutils
@@ -45,6 +50,8 @@ 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 +1,24 @@
+# Keep existing value if it is already non-empty
+if [[ -z "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]] && type gsettings >/dev/null; then
+    echo 'Attempting to get GNOME desktop interface scaling factor' >&2
+    AVALONIA_GLOBAL_SCALE_FACTOR="$(gsettings get org.gnome.desktop.interface scaling-factor)"
+    AVALONIA_GLOBAL_SCALE_FACTOR="${AVALONIA_GLOBAL_SCALE_FACTOR##* }"
+fi
+
+if [[ -z "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]] && type xrdb >/dev/null; then
+    echo 'Attempting to get scaling factor from X FreeType DPI setting' >&2
+    dpi="$(xrdb -get Xft.dpi)"
+    if [[ -n "${dpi}" ]]; then
+        AVALONIA_GLOBAL_SCALE_FACTOR=$(echo "scale=2; ${dpi}/96" | bc)
+    fi
+fi
+
+if [[ "${AVALONIA_GLOBAL_SCALE_FACTOR-}" == "0" ]]; then
+    echo 'Unset invalid scaling value' >&2
+    unset AVALONIA_GLOBAL_SCALE_FACTOR
+fi
+
+if [[ -n "${AVALONIA_GLOBAL_SCALE_FACTOR-}" ]]; then
+    echo "Applying scale factor: ${AVALONIA_GLOBAL_SCALE_FACTOR}" >&2
+    export AVALONIA_GLOBAL_SCALE_FACTOR
+fi