Add GtkGradient.

This type defines a gradient between 2 symbolic colors, with any number
of color stop between these. At the moment it has been put besides
GtkSymbolicColor, although should be completely private, it will likely
need extending in the future for radial gradients.

At some point, it maybe should also be used for images, so painting
both a gradient and an image is consistent.

gtk/gtksymboliccolor.c

@@ -22,6 +22,9 @@
 #include "gtkstyleset.h"
 #include "gtkintl.h"
 
+G_DEFINE_BOXED_TYPE (GtkGradient, gtk_gradient,
+                     gtk_gradient_ref, gtk_gradient_unref)
+
 /* Symbolic colors */
 typedef enum {
   COLOR_TYPE_LITERAL,
@@ -55,6 +58,26 @@ struct GtkSymbolicColor
   };
 };
 
+typedef struct ColorStop ColorStop;
+
+struct ColorStop
+{
+  gdouble offset;
+  GtkSymbolicColor *color;
+};
+
+struct GtkGradient
+{
+  gdouble x0;
+  gdouble y0;
+  gdouble x1;
+  gdouble y1;
+
+  GArray *stops;
+
+  guint ref_count;
+};
+
 GtkSymbolicColor *
 gtk_symbolic_color_new_literal (GdkColor *color)
 {
@@ -240,3 +263,112 @@ gtk_symbolic_color_get_type (void)
 
   return type;
 }
+
+/* GtkGradient */
+GtkGradient *
+gtk_gradient_new_linear (gdouble x0,
+                         gdouble y0,
+                         gdouble x1,
+                         gdouble y1)
+{
+  GtkGradient *gradient;
+
+  gradient = g_slice_new (GtkGradient);
+  gradient->stops = g_array_new (FALSE, FALSE, sizeof (ColorStop));
+
+  gradient->x0 = x0;
+  gradient->y0 = y0;
+  gradient->x1 = x1;
+  gradient->y1 = y1;
+
+  gradient->ref_count = 1;
+
+  return gradient;
+}
+
+void
+gtk_gradient_add_color_stop (GtkGradient      *gradient,
+                             gdouble           offset,
+                             GtkSymbolicColor *color)
+{
+  ColorStop stop;
+
+  g_return_if_fail (gradient != NULL);
+
+  stop.offset = offset;
+  stop.color = gtk_symbolic_color_ref (color);
+
+  g_array_append_val (gradient->stops, stop);
+}
+
+GtkGradient *
+gtk_gradient_ref (GtkGradient *gradient)
+{
+  g_return_val_if_fail (gradient != NULL, NULL);
+
+  gradient->ref_count++;
+
+  return gradient;
+}
+
+void
+gtk_gradient_unref (GtkGradient *gradient)
+{
+  g_return_if_fail (gradient != NULL);
+
+  gradient->ref_count--;
+
+  if (gradient->ref_count == 0)
+    {
+      guint i;
+
+      for (i = 0; i < gradient->stops->len; i++)
+        {
+          ColorStop *stop;
+
+          stop = &g_array_index (gradient->stops, ColorStop, i);
+          gtk_symbolic_color_unref (stop->color);
+        }
+
+      g_array_free (gradient->stops, TRUE);
+      g_slice_free (GtkGradient, gradient);
+    }
+}
+
+gboolean
+gtk_gradient_resolve (GtkGradient      *gradient,
+                      GtkStyleSet      *style_set,
+                      cairo_pattern_t **resolved_gradient)
+{
+  cairo_pattern_t *pattern;
+  guint i;
+
+  g_return_val_if_fail (gradient != NULL, FALSE);
+  g_return_val_if_fail (GTK_IS_STYLE_SET (style_set), FALSE);
+  g_return_val_if_fail (resolved_gradient != NULL, FALSE);
+
+  pattern = cairo_pattern_create_linear (gradient->x0, gradient->y0,
+                                         gradient->x1, gradient->y1);
+
+  for (i = 0; i < gradient->stops->len; i++)
+    {
+      ColorStop *stop;
+      GdkColor color;
+
+      stop = &g_array_index (gradient->stops, ColorStop, i);
+
+      if (!gtk_symbolic_color_resolve (stop->color, style_set, &color))
+        {
+          cairo_pattern_destroy (pattern);
+          return FALSE;
+        }
+
+      cairo_pattern_add_color_stop_rgb (pattern, stop->offset,
+                                        color.red / 65535.,
+                                        color.green / 65535.,
+                                        color.blue / 65535.);
+    }
+
+  *resolved_gradient = pattern;
+  return TRUE;
+}