Fix for recent "Convert to windows .ico format no longer inverts alpha channel." patch

I got this one wrong, in a way which busts conversion to .ico's when  
there is an alpha channel.

I said:
> So around line 997 where we have
>
>          if (next->matte == MagickFalse)
>             *q++=ScaleQuantumToChar(255);
>           else
>             *q++=ScaleQuantumToChar(p->opacity);
>
> I think the last line should be
>             *q++=ScaleQuantumToChar(255 - p->opacity);

but in fact it should be
		*q++=ScaleQuantumToChar(QuantumRange - p->opacity);

255 is of course a char value, but we are converting from a Quantum  
*to* a char.

I've tested this change on some more real world examples.

One thing I still don't understand, though, is the *q+ 
+=ScaleQuantumToChar(255) line, which seems like it should then be
"*q++=ScaleQuantumToChar(QuantumRange)" Perhaps in the next->matte ==  
MagickFalse, the particular value is unimportant, though I've not  
tested that.


Sorry for the confusion!

-Matt