st: Honor alpha range specified by pango_attr_foreground_alpha_new()

st: Honor alpha range specified by pango_attr_foreground_alpha_new()

This is documented as a value between 1 and 65536. However we were passing
a 0 value for 100% transparent colors, which is interpreted as "system
inherited" in pango_renderer_get_alpha() docs.

Ensure we respect this range by specifying the minimum allowed alpha (1)
if the color is fully transparent. If someone notices this 1/65535th change
I'll ask him how many pleiades can he count.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2504
(yes, again).