This created lots and lots of critical warnings when rendering
tests/svg1.1/svg/filters-displace-01-f.svg:

GdkPixbuf-CRITICAL **: gdk_pixbuf_new: assertion `width > 0' failed
g_logv() [gmessages.c:779]
g_log() [gmessages.c:826]
g_return_if_fail_warning() [gmessages.c:838]
gdk_pixbuf_new() [gdk-pixbuf.c:338]
rsvg_filter_primitive_image_render_ext() [rsvg-filter.c:3376]