Fix dmg installer to not have duplicate images and icons

We added a new step in macvim-dev#1540 to generate the image / icon set for the
dmg installer during the dmg build step, but the intermediate files were
erroneously placed in the same folder as the one being copied over,
causing them to be in the final dmg as well. Just use a scratch folder
instead.

src/Makefile

@@ -3717,6 +3717,7 @@ RUNTIME_FOLDER_LIST = MacVim/auto/runtime_folder_list.xcfilelist
 
 RELEASEDIR = MacVim/build/Release
 DMGDIR = MacVim/build/dmg
+DMGIMGDIR = MacVim/build/dmg_images
 DMGFILE = MacVim.dmg
 ENTITLEMENTS = MacVim/MacVim.entitlements
 
@@ -3753,18 +3754,20 @@ macvim-dmg-legacy: macvim-dmg
 #   Finder this will still clip the bottom but there's nothing we can do about
 #   that.
 macvim-dmg:
+	rm -rf $(DMGIMGDIR)
+	mkdir -p $(DMGIMGDIR)
+	tiffutil -cathidpicheck MacVim/dmg/[email protected] MacVim/dmg/[email protected] -out $(DMGIMGDIR)/background.tiff
+	MacVim/scripts/create_icns MacVim/dmg/volume_icons $(DMGIMGDIR) volume
 	rm -rf $(DMGDIR)
 	mkdir -p $(DMGDIR)
 	cp -a $(RELEASEDIR)/MacVim.app $(DMGDIR)/
-	tiffutil -cathidpicheck MacVim/dmg/[email protected] MacVim/dmg/[email protected] -out $(DMGDIR)/background.tiff
-	MacVim/scripts/create_icns MacVim/dmg/volume_icons $(DMGDIR) volume
 	rm -rf $(RELEASEDIR)/$(DMGFILE)
 	MacVim/create-dmg/create-dmg \
 		--filesystem "$(DMGFILESYSTEM)" \
 		--format "$(DMGFORMAT)" \
 		--volname "MacVim" \
-		--volicon $(DMGDIR)/volume.icns \
-		--background $(DMGDIR)/background.tiff \
+		--volicon $(DMGIMGDIR)/volume.icns \
+		--background $(DMGIMGDIR)/background.tiff \
 		--window-size 650 500 \
 		--icon-size 128 \
 		--icon MacVim.app 210 290 \