Image update strategy

[Emanuele Tamponi]

> Isn't it very unlikely that an image contains a completely opaque layer?
> Even if you take the exact bounds there could be holes inside the layer.
> And scanning for the biggest opaque rect could be very slow.

That's true! We don't just have to take into account the layer opacity, but 
contents opacity too.
Well, that's just an optimization tough. If we can't find a good and fast way 
to do this, we can just take into account all layers in a region, instead of 
only the "visible" ones (since, as you said, it can be difficult to say 
*what* is currently visible).

Thank you,
Emanuele