fix(scrollbind): properly take filler/virtual lines into account

Problem:

`'scrollbind'` does not work properly if the window being scrolled
automatically contains any filler/virtual lines (except for diff filler
lines).

This is because when the scrollbind check is done, the logic only
considers changes to topline which are represented as line numbers.

Solution:

Write the logic for determine the scroll amount to take into account
filler/virtual lines.

Fixes #29751

src/nvim/option.c

@@ -88,6 +88,7 @@
 #include "nvim/os/os.h"
 #include "nvim/os/os_defs.h"
 #include "nvim/path.h"
+#include "nvim/plines.h"
 #include "nvim/popupmenu.h"
 #include "nvim/pos_defs.h"
 #include "nvim/regexp.h"
@@ -2474,7 +2475,7 @@ static const char *did_set_scrollbind(optset_T *args)
     return NULL;
   }
   do_check_scrollbind(false);
-  win->w_scbind_pos = win->w_topline;
+  win->w_scbind_pos = get_vtopline(win);
   return NULL;
 }