[racket] bitmap get-argb-pixels returns un-premultiplied alpha?

I've added an argument to `get-argb-pixels' and `set-argb-pixels' to
make them work in premultiplied mode.

At Sat, 23 Apr 2011 22:54:47 +0200, Stephan Houben wrote:
> Hi list,
> 
> Today I noticed that the bitmap's get-argb-pixels method returns
> un-premultiplied alpha, even though it internally uses Cairo
> which uses pre-multiplied alpha.
> 
> Looking at the code, it goes out of its way to convert
> pre-multipled alpha to un-premultiplied alpha.
> I then need to go out of my way again to convert it back to
> premultiplied alpha. Which is inefficient and silly.
> 
> Could we have a method to get at the premultiplied alpha directly?
> The Cairo folks chose it for a reason; if you want to interpolate between
> two pixel values then only premultiplied alpha will give the correct result.
> 
> (See http://www.gimp.org/docs/plug-in/appendix-alpha.html for some discussion 
> on this).
> 
> I would therefore assume that it might be of wider interest than just me.
> 
> Thanks,
> 
> Stephan