Code for loading "CM5" (CPC Mode 5) pixtures

* Fix a bug in the Set_layer function : it was only possible to set a new layer, and this format needs us to write to previous layers too
 * Saving is missing (you need to save as png then convert back to cm5 using SyX converter)
 * Preview does not work (it does not use layers, so the result is broken)
 * Loading switches the "8bit" mode on automatically

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/cpcmode5@1723 416bcca6-2ee7-4201-b75f-2eb2f807beb1

src/const.h

@@ -120,6 +120,7 @@ enum FILE_FORMATS
   FORMAT_KCF,
   FORMAT_PAL,
   FORMAT_SCR,
+  FORMAT_CM5,
   FORMAT_XPM,
   FORMAT_MISC, ///< Must be last of enum: others formats recognized by SDL_image
 };