bug#51490: Show an indicator when Emacs is busy somewhere in the Emacs window

[Gregory Heytings <gregory@heytings.org>]

>>>> Aren't they the simplest graphical element drawn by Emacs, a simple 
>>>> black-and-white bitmap?
>>>
>>> No, they aren't.  They are certainly not black-and-white: they use 
>>> faces.
>>
>> They use faces to get at the colours, but nothing beyond that, I think?
>
> Yes, only the colors.  But accessing the faces means you cannot do that 
> while the Lisp machine is doing something.
>

But would it be really necessary to access these faces each time the 
indicator would be drawn?  We could for example cache the values of these 
two colors somewhere, or decide to do something different and use a 
specific function to set these two colors without going through the whole 
face machinery, something like (set-busy-indicator-colors FOREGROUND 
BACKGROUND).