Improve portability of alpha channel handling on TrueColor visuals

Improve portability of alpha channel handling on TrueColor visuals

* src/xfns.c (select_visual): Always ask for a TrueColor visual.
* src/xfont.c (xfont_draw):
* src/xftfont.c (xftfont_draw): Test `alpha_bits' instead of
n_planes.
* src/xterm.c (x_fill_rectangle, x_clear_rectangle): Likewise.
(x_query_colors, x_alloc_nearest_color): Remove code that
hard-coded alpha mask.
(x_term_init): Calculate alpha bits and offset and populate
field appropriately.
* src/xterm.h (struct x_display_info): New fields `alpha_bits'
and `alpha_offset'.
(x_make_truecolor_pixel): Stop hardcoding the value of the alpha
mask.