initial atari tests (not with image) and dither improvements

palette selection now biases to matches on the bright side of
the RGB color cube, since black is always available for dithering

does better but still not perfect

atari binary just writes some test data to text framebuffer
but builds and runs

Makefile

@@ -0,0 +1,19 @@
+.PHONY : all clean
+
+all : dither4.xex
+
+# sample5.s from sample5.jpg
+
+# reminder: $< is input
+#           $@ is output
+%.o : %.s
+	ca65 -v -t atari -o $@ $<
+
+dither4.xex : dither4.o
+	ld65 -v -C atari-asm-xex.cfg -o $@ $< 
+
+clean :
+	rm -f sample5.s
+	rm -f sample5.o
+	rm -f dither4.o
+	rm -f dither4.xex