(Conditionally) add further dithering algorithms

src/CrossPointSettings.h

@@ -137,12 +137,16 @@ class CrossPointSettings {
 
   // Image rendering in EPUB reader
   enum IMAGE_RENDERING { IMAGES_DISPLAY = 0, IMAGES_PLACEHOLDER = 1, IMAGES_SUPPRESS = 2, IMAGE_RENDERING_COUNT };
+#ifdef ENABLE_IMAGE_DITHERING_EXTENSION
   enum IMAGE_DITHERING {
     IMAGE_DITHER_BAYER = 0,
     IMAGE_DITHER_ATKINSON = 1,
     IMAGE_DITHER_DIFFUSED_BAYER = 2,
     IMAGE_DITHERING_COUNT
   };
+#else
+  enum IMAGE_DITHERING { IMAGE_DITHER_BAYER = 0, IMAGE_DITHERING_COUNT };
+#endif
 
   // Timezone options (POSIX TZ rules for DST support)
   enum TIMEZONE {