Stop referring to ClutterTexture

ClutterTexture is removed from mutter's clutter fork, so lets stop
referring to it. This also happens to fix an incorrect type cast.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/822

src/shell-app.c

@@ -223,7 +223,7 @@ window_backed_app_get_icon (ShellApp *app,
 /**
  * shell_app_create_icon_texture:
  *
- * Look up the icon for this application, and create a #ClutterTexture
+ * Look up the icon for this application, and create a #ClutterActor
  * for it at the given size.
  *
  * Return value: (transfer none): A floating #ClutterActor

src/st/st-drawing-area.c

@@ -22,8 +22,7 @@
  * SECTION:st-drawing-area
  * @short_description: A dynamically-sized Cairo drawing area
  *
- * #StDrawingArea is similar to #ClutterCairoTexture in that
- * it allows drawing via Cairo; the primary difference is that
+ * #StDrawingArea allows drawing via Cairo; the primary difference is that
  * it is dynamically sized. To use, connect to the #StDrawingArea::repaint
  * signal, and inside the signal handler, call
  * st_drawing_area_get_context() to get the Cairo context to draw to.  The

src/st/st-image-content.c

@@ -146,7 +146,7 @@ st_image_content_get_preferred_size (ClutterContent *content,
 {
   StImageContent *self = ST_IMAGE_CONTENT (content);
   StImageContentPrivate *priv = st_image_content_get_instance_private (self);
-  ClutterTexture *texture;
+  CoglTexture *texture;
 
   texture = clutter_image_get_texture (CLUTTER_IMAGE (content));