gdb/tui: avoid extra refresh_window on horizontal scroll

gdb/tui: avoid extra refresh_window on horizontal scroll

While working on the previous patches I noticed that in some cases I
was seeing two calls to tui_source_window_base::refresh_window when
scrolling the window horizontally.

The two calls would trigger in for the tui-disasm-long-lines.exp test
when the pad needed to be refilled.  The two called both come from
tui_source_window_base::show_source_content.  The first call is nested
within check_and_display_highlight_if_needed, while the second call is
done directly at the end of show_source_content.

The check_and_display_highlight_if_needed is being used to draw the
window box to the window, this is needed here because
show_source_content is what gets called when the window needs
updating, e.g. after a resize.  We could potentially do the boxing in
refresh_window, but then we'd be doing it each time we scroll, even
though the box doesn't need changing in this case.

However, we can move the check_and_display_highlight_if_needed to be
the last thing done in show_source_content, this means that we can
rely on the refresh_window call within it to be our single refresh
call.

There should be no user visible changes after this commit.