Removed int casts in the SVG plugin

The casts caused a segfault when drawing an ASTERISK marker,
because %g expects a 64bit double, not a 32bit int.

Change-Id: I7b5ed781bafc596281b604ae48e2eaef598906b5
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/125
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>