Support different generators

Makefile

 SCREENSHOT_DIR := $(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))/.screenshots
+TARGET := demo.pdf
 
-include theme/Makefile
+all: demo
+
+.PHONY: alldemo demo-xelatex demo-lualatex demo-pdflatex
+
+demo-xelatex: LATEXMK_GEN := -xelatex
+demo-lualatex: LATEXMK_GEN := -lualatex
+demo-pdflatex: LATEXMK_GEN := -pdf
 
-.PHONY: demo
+demo-xelatex demo-lualatex demo-pdflatex: clean
 
-demo: demo.pdf
-	mkdir .screenshots
-	pdftoppm -scale-to 500 -f 1 -l 5 -jpeg $< $(SCREENSHOT_DIR)/$@
+demo demo-xelatex demo-lualatex demo-pdflatex: $(TARGET)
+	test -d "$(SCREENSHOT_DIR)" || mkdir "$(SCREENSHOT_DIR)"
+	pdftoppm -scale-to 500 -f 1 -l 5 -png $< "$(SCREENSHOT_DIR)/$@"
 
 mrproper::
-	@rm -f demo.pdf $(SCREENSHOT_DIR)/demo-*.jpg
-	@rmdir $(SCREENSHOT_DIR)
+	rm -f "$(TARGET)" "$(SCREENSHOT_DIR)"/demo-*.png
+	test -d "$(SCREENSHOT_DIR)" && rmdir --ignore-fail-on-non-empty "$(SCREENSHOT_DIR)"
+
+include theme/Makefile
+