fix DPI mixup

librsvg defaulted to 90 DPI, but libvips to 72 DPI -- add a correction
factor

thanks Fosk!

see https://github.com/jcupitt/libvips/issues/688

ChangeLog

@@ -1,6 +1,8 @@
 12/6/17 started 8.5.7
 - transform cmyk->rgb automatically on write if there's an embedded profile
   and the saver does not support cmyk
+- fix DPI mixup in svgload ... we were writing images about 20% too large,
+  thanks Fosk
 
 19/5/17 started 8.5.6
 - tiff read with start page > 0 could break edge tiles or strips

libvips/foreign/svgload.c

@@ -6,6 +6,8 @@
  * 	- add svgz support
  * 18/1/17
  * 	- invalidate operation on read error
+ * 8/7/17
+ * 	- fix DPI mixup, thanks Fosk
  */
 
 /*
@@ -115,6 +117,11 @@ vips_foreign_load_svg_build( VipsObject *object )
 	if( !vips_object_argument_isset( object, "scale" ) )
 		svg->scale = svg->dpi / 72.0;
 
+	/* librsvg defaults to 90 DPI, but vips defaults to 72. We need to
+	 * adjust the scale down.
+	 */
+	svg->scale *= 72.0 / 90.0;
+
 	if( VIPS_OBJECT_CLASS( vips_foreign_load_svg_parent_class )->
 		build( object ) )
 		return( -1 );