[xterm.h] note on 'dragging' field of 'struct scroll_bar'

[Dmitry Antipov <dmantipov@yandex.ru>]

I want to notice about the placement of 'dragging' field within 'struct scroll_bar'.
It's a Lisp_Object, but placed so it's not reachable by the marking traversal. IIUC,
it works just because it's always an integer, Qnil or Qt; but this is a potential trap
for someone who will change this someday. So, I think it's better to use plain C 'int'
instead, or move 'dragging' up to the head of 'struct scroll_bar' before 'x_window'.

And, the more general question is: why not use the same 'struct scroll_bar' both for
X and Win32 code?

Dmitry