Fix app icon fading

The way we were loading data into a CoglTexture, then pulling it out
and manipulating it on the CPU, then loading it back into a texture
was a bit lame.

Clean things up a bit here by loading directly into the CPU, doing
the fading, then creating a texture.

Also cache the faded data in StTextureCache.

https://bugzilla.gnome.org/show_bug.cgi?id=612759

src/shell-app-system.c

@@ -1021,7 +1021,16 @@ themed_icon_from_name (const char *iconname)
   return icon;
 }
 
-static GIcon *
+/**
+ * shell_app_info_get_icon:
+ * @info: A #ShellAppInfo
+ *
+ * Get the #GIcon associated with this app; for apps "faked" from a #MetaWindow,
+ * return %NULL.
+ *
+ * Returns: (transfer full): The icon for @info, or %NULL
+ */
+GIcon *
 shell_app_info_get_icon (ShellAppInfo *info)
 {
   char *iconname = NULL;
@@ -1120,6 +1129,21 @@ shell_app_info_create_icon_texture (ShellAppInfo *info, float size)
   return ret;
 }
 
+/**
+ * shell_app_info_get_source_window:
+ * @info: A #ShellAppInfo
+ *
+ * If @info is tracking a #MetaWindow, return that window.
+ * Otherwise, return %NULL.
+ */
+MetaWindow *
+shell_app_info_get_source_window (ShellAppInfo *info)
+{
+  if (info->type == SHELL_APP_INFO_TYPE_WINDOW)
+    return info->window;
+  return NULL;
+}
+
 /**
  * shell_app_info_launch_full:
  * @timestamp: Event timestamp, or 0 for current event timestamp