compositor: Document options for headless compositor

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>

[Pekka: s/Wayland surface/memory surface/, and say default renderer.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.c

@@ -4595,6 +4595,16 @@ usage(int error_code)
 		"\n");
 #endif
 
+#if defined(BUILD_HEADLESS_COMPOSITOR)
+	fprintf(stderr,
+		"Options for headless-backend.so:\n\n"
+		"  --width=WIDTH\t\tWidth of memory surface\n"
+		"  --height=HEIGHT\tHeight of memory surface\n"
+		"  --transform=TR\tThe output transformation, TR is one of:\n"
+		"\tnormal 90 180 270 flipped flipped-90 flipped-180 flipped-270\n"
+		"  --use-pixman\t\tUse the pixman (CPU) renderer (default: no rendering)\n\n");
+#endif
+
 	exit(error_code);
 }