Enable scrolling optimization for xwidgets

Enable scrolling optimization for xwidgets

* src/dispextern.h (struct glyph): Store xwidget ID instead of a
reference.
* src/dispnew.c (scrolling_window): Enable scrolling optimization on
xwidget builds.
* src/xdisp.c (fill_xwidget_glyph_string, produce_xwidget_glyph):
Obtain xwidget from ID.
* src/xterm.c (x_scroll_run): Scroll xwidget windows.
* src/xwidget.c (id_to_xwidget_map, xwidget_counter): New xwidget
variables.
(Fmake_xwidget): Assign each xwidget a unique ID, and keep track of
that ID.
(xwidget_from_id): New function.
(syms_of_xwidget): Initialize id_to_xwidget_map.
(xwidget_end_redisplay): Lookup xwidgets via ID.
* src/xwidget.h (struct xwidget): Add ID field.
(xwidget_from_id): New function.