Allow loading prerendered images for blending etc

[randomjunk]

Mapnik bindings currently do not allow you to load an image from a file.

Being able to do this is useful if you have a prerendered image which needs blending with the mapnik output. (ie: in my example, we are rendering contours, of which there are a large number, but they never change, so rather than render them every time we want to render them once and then re-use the transparent PNGs produced)

A patch to allow this is attached.

[randomjunk]

patch to allow image loading from bindings

[artem]

I was planning to add 'loading images' for a while. Your patch prompt me to do it, thanks:)

>>> im = Image.open("map.png")

Use 'views' if you need to access just a portion of an image.