Fix more uses of opaque Visual structure

Fix more uses of opaque Visual structure

* src/image.c (x_kill_gs_process):
* src/xfaces.c (x_free_colors):
(x_free_dpy_colors):
* src/xfns.c (Fxw_display_color_p):
(Fx_display_grayscale_p):
(Fx_display_visual_class):
* src/xterm.c (x_copy_color):
* src/xterm.h (x_mutable_colormap): Stop using private fields of
Visual.