[layers] Implemented layer insertion. No keys or menu yet, but it's already used when loading images, to make an image with right number of layers. Undoing/redoing such change works. Implemented single-layer deletion (untested)

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/layers@1069 416bcca6-2ee7-4201-b75f-2eb2f807beb1

loadsave.c

@@ -460,7 +460,7 @@ void Init_preview(short width,short height,long size,int format, enum PIXEL_RATI
   {
     if (Pixel_load_function==Pixel_load_in_current_screen)
     {
-      if (Backup_with_new_dimensions(0,width,height))
+      if (Backup_with_new_dimensions(0,1,width,height))
       {
         // La nouvelle page a pu être allouée, elle est pour l'instant pleine
         // de 0s. Elle fait Main_image_width de large.
@@ -3174,6 +3174,7 @@ void Load_GIF(void)
               if (number_LID!=0)
               {
                 Main_current_layer++;
+                Add_layer(Main_backups, Main_current_layer);
               }
               number_LID++;