Add support for ARGB visuals on non-Xt builds

Add support for ARGB visuals on non-Xt builds

* src/gtkutil.c (xg_create_frame_widgets): Enable RGBA visual
on non-PGTK builds.
* src/xfns.c (select_visual): Select 32-bit visuals on non-Xt
builds.
* src/xterm.c (x_draw_fringe_bitmap):
(x_draw_image_glyph_string): Stop using DefaultDepthOfScreen.
(x_query_colors):
* src/xterm.h (x_make_truecolor_pixel): Make colors opaque on
32-bit visuals.