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/pages.c

@@ -3,7 +3,7 @@
 /*  Grafx2 - The Ultimate 256-color bitmap paint program
 
     Copyright 2008 Franck Charlet
-    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
@@ -1531,6 +1531,7 @@ void Switch_layer_mode(enum IMAGE_MODES new_mode)
   {
     case IMAGE_MODE_MODE5:
     case IMAGE_MODE_LAYERED:
+    default:
       Update_buffers(Main_image_width, Main_image_height);
       Redraw_layered_image();
       break;