Re-enable some pdf tests now that poppler has knockout support

We had several pdf tests disabled waiting for this bug fix:

	Poppler does not correctly handle knockout groups
	https://bugs.freedesktop.org/show_bug.cgi?id=12185

That's in place for poppler now, so we're turning the tests
back on. Some of the affected tests now pass perfectly:

	over-above-source
	over-around-source
	over-below-source
	over-between-source

Some just needed new reference images:

	operator-clear
	clip-operator-pdf-argb32

The remaining tests still fail, but none of the failures can
obviously be ascribed to just poppler problems:

	clip-operator-pdf-rgb24
	operator-source
	unbounded-operator

The first two have some serious problems, while in the case
of unbounded-operator the problem is extremely minor (a white
grid appears in the background where the reference image is
all black).

boilerplate/cairo-boilerplate-pdf.c

@@ -77,19 +77,6 @@ static const char *pdf_ignored_tests[] = {
     "radial-gradient",
     "text-pattern",
     "trap-clip",
-    /* These next 7 failures are due to:
-     *
-     *	Poppler does not correctly handle knockout groups
-     *	https://bugs.freedesktop.org/show_bug.cgi?id=12185
-     */
-    "clip-operator",
-    "operator-clear",
-    "operator-source",
-    "over-above-source",
-    "over-around-source",
-    "over-below-source",
-    "over-between-source",
-    "unbounded-operator"
 };
 
 cairo_surface_t *