bug#4033: 23.1; list-colors-display is misleading

[Drew Adams]

> > IOW, even if my particular display only supports 2 hex digits 
> > per component, if Emacs supports 16 bits per component
> > internally, doesn't that mean that it supports up to
> > #RRRRGGGGBBBB?
> 
> It supports that, but it cannot actually display that if the display
> driver has only 8 bits per component.  I.e., the extra bits get lost
> when the color is actually displayed.

I think that's what I just said, so it sounds like I'm on the same page:
internally 16 bits, but a given display might be more limited.

The idea then is to have Emacs use an RGB format that is appropriate for the
user's display. If the display supports 8 bits per component, it would show
#RRGGBB. If the display supports 16 bits per component, it would show
#RRRRGGGGBBBB. If the display supports 12 bits per component, it would show
#RRRGGGBBB. 

Whatever the display's support, the user would see the most appropriate RGB
format. See the code I sent, which I think does that.