Keep track of scale factor by atimer and recreate cairo_surface_t

Keep track of scale factor by atimer and recreate cairo_surface_t

Otherwise texts become blurry when a frame moved from 1x monitor
to 2x monitor.  I need GTK's such signal, but there isn't. Instead
I watch frame's monitor's scale factor periodically.  We can see
blurriness for a short time, but it is gone soon.

* src/pgtkfns.c (update_watched_scale_factor): New function to check
scale factor and recreate cairo_surface_t if changed.
* src/pgtkfns.c (Fx_create_frame): Initialize atimer.
(Fx_show_tip): Add an argument.
* src/pgtkterm.c (FRAME_CR_SURFACE_DESIRED_WIDTH): Move macros to pgtkterm.h
(x_free_frame_resources): Free atimer.
(size_allocate): Add an argument.
(pgtk_cr_update_surface_desired_size): Add an argument. Recreate if it
is true.
* src/pgtkterm.h (struct pgtk_output): New members.
(FRAME_CR_SURFACE_DESIRED_HEIGHT): Move macros from pgtkterm.c