doc: enable interactive SVG output

Let's not target IE8. SVG graphs are much more comfortable to read and
scale, and interactive SVG allows more comfortable viewing of big
graphs.

doc/Doxyfile.in

@@ -2276,7 +2276,7 @@ DIRECTORY_GRAPH        = YES
 # The default value is: png.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-DOT_IMAGE_FORMAT       = png
+DOT_IMAGE_FORMAT       = svg
 
 # If DOT_IMAGE_FORMAT is set to svg, then this option can be set to YES to
 # enable generation of interactive SVG images that allow zooming and panning.
@@ -2288,7 +2288,7 @@ DOT_IMAGE_FORMAT       = png
 # The default value is: NO.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-INTERACTIVE_SVG        = NO
+INTERACTIVE_SVG        = YES
 
 # The DOT_PATH tag can be used to specify the path where the dot tool can be
 # found. If left blank, it is assumed the dot tool can be found in the path.