desktopToDarwinBundle: fix 16x, 32x app icons

Changes the script to produce rgb+mask images for 16x and 32x icons
instead of png.

Using icns files containing 16x and 32x png images for app bundles
results in Finder misrendering icons for these sizes (even though the
icns files are rendered correctly when viewed by themselves)

pkgs/build-support/setup-hooks/desktop-to-darwin-bundle.sh

···
 
         echo "desktopToDarwinBundle: resizing icon $in to $out, size $dim" >&2
         magick convert -scale "${dim}x${dim}" -density "$density" -units PixelsPerInch "$in" "$out"
+        convertIfUnsupportedIcon "$out" "$iconSize" "$scale"
     }
 
     function synthesizeIcon() {
···
             echo "desktopToDarwinBundle: rasterizing svg $in to $out, size $dim" >&2
             rsvg-convert --keep-aspect-ratio --width "$dim" --height "$dim" "$in" --output "$out"
             magick convert -density "$density" -units PixelsPerInch "$out" "$out"
+            convertIfUnsupportedIcon "$out" "$iconSize" "$scale"
         else
             return 1
+        fi
+    }
+
+    # macOS does not correctly display 16x and 32x png icons on app bundles
+    # they need to be converted to rgb+mask (argb is supported only from macOS 11)
+    function convertIfUnsupportedIcon() {
+        local -r in=$1
+        local -r iconSize=$2
+        local -r scale=$3
+        local -r out=${in%.png}.rgb
+
+        if [[ ($scale -eq 1) && ($iconSize -eq 32 || $iconSize -eq 16) ]]; then
+            echo "desktopToDarwinBundle: converting ${iconSize}x icon to rgb" >&2
+            icnsutil convert "$out" "$in"
+            rm "$in"
         fi
     }
 
···
                     fixed)
                         local density=$((72 * scale))x$((72 * scale))
                         magick convert -density "$density" -units PixelsPerInch "$icon" "$result"
+                        convertIfUnsupportedIcon "$result" "$iconSize" "$scale"
                         foundIcon=OTHER
                         ;;
                     threshold)