Call Button_Quit and Button_Page with correct enum

These functions are called explicitly in a few places and need to be
handed the corresponding enum value to correctly get the button value.
Since they were previously called with -1, they would cause segfaults.
These seemed to happen when restoring backups or loading images to both
pages.

src/main.c

@@ -969,7 +969,7 @@ int Init_program(int argc,char * argv[])
           Redraw_layered_image();
           End_of_modification();
   
-          Button_Page(-1);
+          Button_Page(BUTTON_PAGE);
           // no break ! proceed with the other file now
         case 1:
           Init_context_layered_image(&context, main_filename, main_directory);