bicubic interpolating zoom

[Michael Thaler]

Hello,

I just implemented a very simple bilinear scaling algorith. It is a very 
simple algorithm, but the resulting image does not look too bad if the scale 
factor is not too small or too large (e.g. if it is in the range 0.5 - 2). I 
attached the diff.

However I discovered some problems:

* the code is much slower then Boudewijn's old algorithm
* if you scale an image that is bigger then the visible part of the screen, 
there will be some artefacts: rectangular black regions that are sometimes 
filled with garbage. If you scale an image down so that it is smaller then 
the visible part of the screen, you cannot make them go away. If you scale it 
up you can make these artefacts go away by moving the visible part of the 
image.

Can someone check if this also happens with the old algorithm?

I will try to find out how scaling is done in the GIMP. However, I downloaded 
the source code of the GIMP yesterday and glanced over it and I wasn't able 
to find the relevant code. I think I will need some time to find the relevant 
parts.

Greetings,
Michael
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