Implement a more generic "rasters" mode for CPC

This is similar to mode 5, but with less constraints:
- All inks can have split rasters
- Split rasters have a min width of 32 pixels

Note that this is nowhere near enough to make sure the image will be
renderable (number of colors should be limited, unless there is a span
without rasters allowing to reload registers with new colors, and
moving from one pen to another also has a cost). The goal is to offer no
over-restriction (everything that can be shown on the real machine is
drawable). Additional constraints may be checked from a lua script that
will generate the code and data for displaying the picture.

src/const.h

@@ -614,6 +614,7 @@ enum IMAGE_MODES
   IMAGE_MODE_EGX,       ///< CPC EGX
   IMAGE_MODE_EGX2,      ///< CPC EGX2
   IMAGE_MODE_MODE5,     ///< CPC mode 5
+  IMAGE_MODE_RASTER,    ///< CPC generic rasters
 };
 
 /// Circle / Ellipse Modes