PDF: Fix regression in EXTEND_NONE gradients

The test for opaque gradients in _cairo_pdf_surface_add_pdf_pattern()
must be identical to the test in
_cairo_pdf_surface_emit_pattern_stops() other wise the PDF file will
reference a smask that does not exist.

The _cairo_pattern_is_opaque() test is too strict for PDF as PDF can
draw EXTEND_NONE gradients with opaque color stops without requiring a
smask.

src/cairo-pdf-surface.c

@@ -1204,6 +1204,18 @@ _cairo_pdf_surface_add_source_surface (cairo_pdf_surface_t	*surface,
     return status;
 }
 
+static cairo_bool_t
+_gradient_stops_are_opaque (const cairo_gradient_pattern_t *gradient)
+{
+    unsigned int i;
+
+    for (i = 0; i < gradient->n_stops; i++)
+	if (! CAIRO_COLOR_IS_OPAQUE (&gradient->stops[i].color))
+	    return FALSE;
+
+    return TRUE;
+}
+
 static cairo_status_t
 _cairo_pdf_surface_add_pdf_pattern (cairo_pdf_surface_t		*surface,
 				    const cairo_pattern_t	*pattern,
@@ -1237,7 +1249,8 @@ _cairo_pdf_surface_add_pdf_pattern (cairo_pdf_surface_t		*surface,
     if (pattern->type == CAIRO_PATTERN_TYPE_LINEAR ||
         pattern->type == CAIRO_PATTERN_TYPE_RADIAL)
     {
-        if (_cairo_pattern_is_opaque (pattern, extents) == FALSE) {
+	cairo_gradient_pattern_t *gradient = (cairo_gradient_pattern_t *) pattern;
+	if (! _gradient_stops_are_opaque (gradient)) {
             pdf_pattern.gstate_res = _cairo_pdf_surface_new_object (surface);
 	    if (pdf_pattern.gstate_res.id == 0) {
 		cairo_pattern_destroy (pdf_pattern.pattern);