Add support for transparent gradients

This is based on the gradient patch written by Miklós Erdélyi at
http://lists.freedesktop.org/archives/cairo/2006-August/007648.html

Currently only EXTEND_NONE and EXTEND_PAD are supported. Other extend
types will go through the image fallback path.

src/cairo-pdf-surface-private.h

@@ -65,6 +65,9 @@ struct _cairo_pdf_surface {
     cairo_array_t xobjects;
     cairo_array_t streams;
     cairo_array_t alphas;
+    cairo_array_t smasks;
+    cairo_array_t rgb_linear_functions;
+    cairo_array_t alpha_linear_functions;
 
     cairo_scaled_font_subsets_t *font_subsets;
     cairo_array_t fonts;
@@ -81,6 +84,16 @@ struct _cairo_pdf_surface {
         cairo_output_stream_t *old_output;
     } current_stream;
 
+    struct {
+        cairo_pattern_type_t type;
+        double red;
+        double green;
+        double blue;
+        int alpha;
+        cairo_pdf_resource_t smask;
+        cairo_pdf_resource_t pattern;
+    } emitted_pattern;
+
     cairo_bool_t has_clip;
 
     cairo_paginated_mode_t paginated_mode;