bgo#760180: handle the viewBox of markers correctly

One of the SVG test suite's images has a marker with a viewBox that doesn't actually start at (0, 0).
This was rendered incorrectly.

I'm not sure why this specific use of rsvg_preserve_aspect_ratio() is not the same as the others
in librsvg, but it certainly produced incorrect results.

https://bugzilla.gnome.org/show_bug.cgi?id=760180