Add poppler-specific reference image for radial-gradient

This is justified by the previously mentioned bug on poppler
gradients. Note that this test only passes with a patch
to fix a bug in poppler (not yet upstream). Here it is:

	PATCH: Keep 'cairo_shape' and 'cairo' consistent
	https://bugs.freedesktop.org/show_bug.cgi?id=14593

test/Makefile.am

@@ -462,6 +462,7 @@ REFERENCE_IMAGES = \
 	push-group-rgb24-ref.png	\
 	push-group-svg-argb32-ref.png	\
 	radial-gradient-ref.png	\
+	radial-gradient-pdf-ref.png \
 	radial-gradient-svg-ref.png	\
 	random-intersections-ref.png	\
 	random-intersections-ps-ref.png \

test/README

@@ -214,6 +214,7 @@ linear-gradient
 trap-clip
 linear-gradient
 linear-gradient-reflect
+radial-gradient
 
 Poppler renders patterned text as black
 https://bugs.freedesktop.org/show_bug.cgi?id=14577