Adjustments to double buffering on MS Windows

Adjustments to double buffering on MS Windows

* src/w32fns.c (w32_set_inhibit_double_buffering): Add comment
describing double buffering.
(w32_wnd_proc): Respect `w32-disable-double-buffering'.
(globals_of_w32fns): New variable
`w32-disable-double-buffering'.

* src/w32term.c (w32_show_back_buffer): Return immediately if
double buffering is disabled on the frame.
(w32_scroll_run): Use old scrolling code if
`w32-disable-double-buffering' is enabled.
(w32_scroll_bar_clear): Document why we don't clear scroll bars
when double buffering is enabled.
(w32_read_socket): Respect `w32-disable-double-buffering' and
clean up some code.

* src/w32xfns.c (get_frame_dc): Respect
`w32-disable-double-buffering'.