Move SVGImage import into conditional stagement

SVG image support is optional. By importing the svgimage module at the
top of the image.py module, any rst document that contained an image
would rely on having svglib installed. This commit fixes that problem.

rst2pdf/image.py

@@ -13,8 +13,6 @@
 from .opt_imports import PILImage, pdfinfo
 from .log import log, nodeid
 
-from .svgimage import SVGImage
-
 # This assignment could be overridden by an extension module
 VectorPdf = None
 
@@ -193,6 +191,7 @@ def get_backend(self, uri, client):
 
         if extension in ['svg','svgz']:
             log.info('Backend for %s is SVGIMage'%filename)
+            from .svgimage import SVGImage
             backend=SVGImage
 
         elif extension in ['pdf']:
@@ -246,6 +245,7 @@ def size_for_node(self, node, client):
         xdpi, ydpi = client.styles.def_dpi, client.styles.def_dpi
         extension = imgname.split('.')[-1].lower()
         if extension in ['svg','svgz']:
+            from .svgimage import SVGImage
             iw, ih = SVGImage(imgname, srcinfo=srcinfo).wrap(0, 0)
             # These are in pt, so convert to px
             iw = iw * xdpi / 72