EGX mode for CPC

In this custom mode, odd lines use MODE 0 (wide pixels, 16 colors), and
even lines use MODE 1 (square pixels, 4 colors).

- Add a settings window to the 8-bit effect where you can chose what you
want to draw
- Add the customizations required to make it work in graph.c
- Tweak some other places so it is easier to add other modes later on.
- Groundwork for some more modes: EGX2, ZX spectrum, Game Boy color.

src/palette.c

@@ -2,7 +2,7 @@
 */
 /*  Grafx2 - The Ultimate 256-color bitmap paint program
 
-    Copyright 2007 Adrien Destugues
+    Copyright 2007-2017 Adrien Destugues
     Copyright 1996-2001 Sunset Design (Guillaume Dorme & Karl Maritaud)
 
     Grafx2 is free software; you can redistribute it and/or
@@ -344,7 +344,8 @@ void Remap_image_highlevel(byte * conversion_table)
   int layer;
 
   // Remap the flatenned image view
-  if (Main_backups->Pages->Image_mode == IMAGE_MODE_LAYERED)
+  if (Main_backups->Pages->Image_mode != IMAGE_MODE_ANIMATION
+      && Main_backups->Pages->Image_mode != IMAGE_MODE_MODE5)
   {
     Remap_general_lowlevel(conversion_table,Main_visible_image.Image,Main_visible_image.Image,
                          Main_image_width,Main_image_height,Main_image_width);