st/spinner-content: Fix CoglTexture leak

st/spinner-content: Fix CoglTexture leak

Most of the members were getting indirectly freed via the
st_spinner_content_set_actor (spinner, NULL) and the subsequent
st_spinner_content_invalidate () on destruction of the actor or
changing of its content when the spinner is stopped.
The texture however was not freed.

This now frees all members explicitly on finalize.

208 (96 direct, 112 indirect) bytes in 1 blocks are definitely lost in loss record 20,960 of 22,464
   at 0x4D37833: g_type_create_instance (gtype.c:1877)
   by 0x4D1C186: g_object_new_internal.part.0 (gobject.c:2666)
   by 0x4D1E0FA: g_object_new_internal (gobject.c:2982)
   by 0x4D1E0FA: g_object_new_valist (gobject.c:3004)
   by 0x4D1E488: g_object_new (gobject.c:2479)
   by 0x5A8B7CB: _cogl_texture_2d_create_base (cogl-texture-2d.c:84)
   by 0x572D668: st_spinner_content_paint_content (st-spinner-content.c:175)
   by 0x50D2C07: clutter_actor_paint_node (clutter-actor.c:3471)
   by 0x50D2C07: clutter_actor_continue_paint (clutter-actor.c:3726)
   by 0x511D4F9: clutter_paint_node_paint (clutter-paint-node.c:867)
   by 0x50D9717: clutter_actor_paint (clutter-actor.c:3670)
   by 0x574199A: st_viewport_paint (st-viewport.c:483)
   by 0x50D2C42: clutter_actor_continue_paint (clutter-actor.c:3728)
   by 0x511D4F9: clutter_paint_node_paint (clutter-paint-node.c:867)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4045>
(cherry picked from commit ba0c4bda8083b0190ce5e0f38255b6e7b6fcffcf)