Fix crash in loading svg file from blender

The svg file blender writes out contains a viewBox with wrong content

<svg width="2048px" height="2048px" viewBox="0px 0px 2048px 2048px"
xmlns="http://www.w3.org/2000/svg" version="1.1">

Workaround the bug by removing the px from the viewBox string before loading
the viewBox values.