Explicitly specify icon size instead of density

Instead of calculating a density that we hope results in the correct
size, we can specify what size we want. This is more robust and easier
to understand. This also allows us to simplify the Makefile quite a bit.

Note that Fedora's packaging of ImageMagick has a bug here:
https://bugzilla.redhat.com/show_bug.cgi?id=2140018

app/images/icons/Makefile

@@ -13,23 +13,18 @@ ALL_ICONS := \
 
 all: $(ALL_ICONS)
 
-novnc-16.png: novnc-icon-sm.svg
+# General conversion
-	convert -density 90 \
-		-background transparent "$<" "$@"
-novnc-24.png: novnc-icon-sm.svg
-	convert -density 135 \
-		-background transparent "$<" "$@"
-novnc-32.png: novnc-icon-sm.svg
-	convert -density 180 \
-		-background transparent "$<" "$@"
-
 novnc-%.png: novnc-icon.svg
-	convert -density $$[`echo $*` * 90 / 48] \
+	convert -depth 8 -background transparent \
-		-background transparent "$<" "$@"
+		-size $*x$* "$(lastword $^)" "$@"
 
+# iOS icons use their own SVG
 novnc-ios-%.png: novnc-ios-icon.svg
 	convert -depth 8 -background transparent \
-		-size $*x$* "$<" "$@"
+		-size $*x$* "$(lastword $^)" "$@"
+
+# The smallest sizes are generated using a different SVG
+novnc-16.png novnc-24.png novnc-32.png: novnc-icon-sm.svg
 
 clean:
 	rm -f *.png