[layers] Implemented SwapUp and SwapDown (Alt-PgUp, Alt-PgDown) to change layers order; Implemented layer merge (Alt-End); improved GIF loader to read images with optimized layers (smaller) and the ones with a transparent color different than zero.

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/layers@1077 416bcca6-2ee7-4201-b75f-2eb2f807beb1

pages.h

@@ -41,6 +41,8 @@ void Upload_infos_page_main(T_Page * page);
 byte Add_layer(T_List_of_pages *list, byte layer);
 /// Delete a layer from the latest page of a list. Returns 0 on success.
 byte Delete_layer(T_List_of_pages *list, byte layer);
+/// Merges the current layer onto the one below it.
+byte Merge_layer();
 
 // private
 T_Page * New_page(byte nb_layers);