Comment 3 for bug 517082

Here is a first try, using the bitmap extension system.
It crops a bitmap (or a selection of bitmaps) according to top, bottom, left and right values (in px).

The only drawback is that the bitmap is resized, but not its SVG element. Thus it works well if the height and width SVG attributes are not set. If they are, the image is cropped, but keeps its original size on canvas.

I see no way to resize the SVG element from the extension, except changing things in extension/internal/bitmap/imagemagick.cpp.