css: Make image values resolve used values

Separate out computed and used values for this type.

gtk/gtkcssimagevalue.c

@@ -143,6 +143,7 @@ _gtk_css_image_value_new (GtkCssImage *image)
   value = gtk_css_value_new (GtkCssValue, &GTK_CSS_VALUE_IMAGE);
   value->image = image;
   value->is_computed = gtk_css_image_is_computed (image);
+  value->contains_current_color = gtk_css_image_contains_current_color (image);
 
   return value;
 }
@@ -155,3 +156,13 @@ _gtk_css_image_value_get_image (const GtkCssValue *value)
   return value->image;
 }
 
+GtkCssValue *
+gtk_css_image_value_resolve (GtkCssValue          *value,
+                             GtkCssComputeContext *context,
+                             GtkCssValue          *current_color)
+{
+  if (!gtk_css_value_contains_current_color (value))
+    return gtk_css_value_ref (value);
+
+  return _gtk_css_image_value_new (gtk_css_image_resolve (_gtk_css_image_value_get_image (value), context, current_color));
+}

gtk/gtkcssimagevalueprivate.h

@@ -28,6 +28,10 @@ GtkCssValue *   _gtk_css_image_value_new           (GtkCssImage         *image);
 
 GtkCssImage *   _gtk_css_image_value_get_image     (const GtkCssValue   *image);
 
+GtkCssValue *    gtk_css_image_value_resolve       (GtkCssValue          *value,
+                                                    GtkCssComputeContext *context,
+                                                    GtkCssValue          *current_color);
+
 
 G_END_DECLS