gdk_window_begin_paint_region() - always reuse the same surface

When using the implicit paint pixmap, always draw to the same surface
(the standard one for the pixmap) each time rather than creating a new
one each time. This is both more effective and more natural.

It also fixes a redraw issue on win32, where using multiple surfaces
on the same HDC sometimes causes issues.  This seems to be due to
leftover state on the HDC from previous surfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=741060